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Mass Percent Calculator

Ready to calculate
% w/w = m_part / m_total × 100.
3 Modes.
% / ppm / ppb.
100% Free.
No Data Stored.

How it Works

01Pick a Mode

Solute in solution, chemical inside a mixture, or percent composition from a molecular formula.

02Enter Masses (or Atom Counts)

For solute mode, enter any 2 of 3 (solute / solvent / solution). For composition, pick elements and counts.

03Apply % w/w = m_part / m_total × 100

Mass solution = solute + solvent. Percent composition uses molar_mass = Σ(count × atomic_weight).

04Get % w/w + ppm + Breakdown

Mass percent, fraction, ppm, ppb — plus per-element percentages and a transparent calculation trace.

What is a Mass Percent Calculator?

Mass percent (also called mass fraction × 100 or % w/w) is one of the most-used concentration units in chemistry — it expresses the mass of a component as a fraction of the total mass of the system, multiplied by 100. The defining identity is % w/w = (mass of part / total mass) × 100. Our Mass Percent Calculator implements all three classic interpretations of this formula in a single tool, matching the way real chemistry problems are framed.

Mode 1 — Mass percent for a solute. Enter any 2 of 3 fields (mass of solute, mass of solvent, mass of solution); the calculator solves for the third using the constraint m_solution = m_solute + m_solvent and reports % w/w along with fraction, ppm, and ppb. Mode 2 — Mass percent for a chemical inside a mixture. Enter mass of chemical and total mass of the compound or sample for direct mass-fraction calculations on alloys, ores, drug content in tablets, fat content in foods, gold purity in karats, and any mixture-composition problem. Mode 3 — Percent composition from a molecular formula. Pick up to 6 elements from a built-in periodic table (37 most-common elements, IUPAC 2021 atomic weights) and enter the atom count for each (e.g. glucose C₆H₁₂O₆ → 6 C, 12 H, 6 O); the calculator computes molar mass = Σ(count × atomic_weight) and the mass percent of every element with a visual percent-by-element bar.

All masses accept g / mg / µg / kg / lb / oz; all results are expressed simultaneously in % w/w, fraction, ppm, and ppb so you can pick whichever your protocol needs. Designed for chemistry students learning stoichiometry, analytical chemists running combustion analysis or assays, pharmacists labeling tablet potency, metallurgists grading alloys, food scientists computing macronutrient percentages, and environmental analysts working with trace contaminants — the tool runs entirely in your browser, no account, no data stored.

Pro Tip: Pair this with our Molarity Calculator for solution preparation, our Grams to Moles Calculator for stoichiometry, or our Dilution Factor Calculator for serial dilution.

How to Use the Mass Percent Calculator?

Pick a Mode: Three radio options. Choose "mass percentage for a solute" for typical solution chemistry (sugar in water, salt in saline). Choose "mass percentage for a chemical" for the mass fraction of any constituent in a mixture (gold in an alloy, active ingredient in a tablet). Choose "percent composition" for the elemental breakdown of a pure compound from its molecular formula.
For Solute Mode — Enter Any 2 of 3 Mass Fields: Mass of solute, mass of solvent, and mass of solution. The calculator solves for the third using m_solution = m_solute + m_solvent. Common workflow: weigh out the solute, weigh out the solvent → calculator gives mass of solution and % w/w.
For Chemical Mode — Enter Both Masses: Mass of the target chemical and total mass of the mixture or compound. Direct division gives the mass fraction.
For Composition Mode — Pick Elements and Enter Atom Counts: Use the dropdowns to pick up to 6 elements; enter the number of atoms of each (e.g. for glucose C₆H₁₂O₆: 6 C, 12 H, 6 O — leave the rest blank). The calculator looks up the IUPAC atomic weight, computes molar mass, and the mass percent of every element with a visual bar.
Apply % w/w = (m_part / m_total) × 100: The calculator converts all masses to grams internally and applies the formula. For percent composition, the formula is %_element = (count × atomic_weight / molar_mass) × 100.
Read Results in 4 Equivalent Forms: % w/w, decimal fraction, ppm (parts per million = % × 10,000), and ppb (parts per billion = % × 10⁷). Pick whichever your reference protocol or regulatory document uses. Use ppm for trace concentrations (drinking water lead 10 ppb, fluoride 0.7 ppm) and % for bulk composition.
Cross-Check the Calculation Breakdown: The result panel shows every intermediate step — input masses in g, derived sum, division, multiplication by 100. Verify against your textbook example or reference protocol.

How is mass percent calculated?

Mass percent is the most temperature-stable concentration unit — because it has no volume term, it doesn't drift with thermal expansion. That makes it the standard for food labeling, alloy grading, drug labeling, and any application where the sample sees a wide temperature range.

Standard analytical chemistry; IUPAC Compendium of Chemical Terminology (Gold Book): "mass fraction"; standard food labeling regulations (FDA, EFSA).

Core Formula

Mass percent (% w/w) = (mass of part / total mass) × 100

For a solution, "total mass" means mass of solute PLUS mass of solvent: m_solution = m_solute + m_solvent. Dividing by mass of solvent only is a frequent error — it inflates the value for dilute solutions and gives 100% (not 50%) when solute equals solvent.

Worked Example — Saline Solution

Dissolve 9 g of NaCl in 991 g of water. What is the mass percent?

  • Mass of solute = 9 g.
  • Mass of solvent = 991 g.
  • Mass of solution = 9 + 991 = 1000 g.
  • % w/w = (9 / 1000) × 100 = 0.9% w/w. (This is "physiological saline" — isotonic with blood plasma.)

Worked Example — Percent Composition of Glucose

Glucose C₆H₁₂O₆. Atomic weights: C = 12.011, H = 1.008, O = 15.999.

  • Mass of C = 6 × 12.011 = 72.066 g/mol.
  • Mass of H = 12 × 1.008 = 12.096 g/mol.
  • Mass of O = 6 × 15.999 = 95.994 g/mol.
  • Molar mass = 72.066 + 12.096 + 95.994 = 180.156 g/mol.
  • % C = (72.066 / 180.156) × 100 = 40.00%.
  • % H = (12.096 / 180.156) × 100 = 6.71%.
  • % O = (95.994 / 180.156) × 100 = 53.29%.
  • Sum: 40.00 + 6.71 + 53.29 = 100.00% ✓.

Worked Example — Alloy Composition

A 100 g brass sample contains 65 g copper and 35 g zinc. What is the % copper?

  • % Cu = (65 / 100) × 100 = 65% w/w copper.
  • % Zn = (35 / 100) × 100 = 35% w/w zinc.
  • This is "yellow brass" — typical for plumbing and decorative castings. (Cartridge brass is 70/30; muntz metal is 60/40.)

Common Mass Percentages You Should Know

  • 0.9% NaCl (physiological saline): 9 g NaCl per 1000 g solution; isotonic with plasma.
  • 3% hydrogen peroxide: household first-aid antiseptic (97% water).
  • 5% acetic acid: household white vinegar.
  • 37% w/w concentrated HCl: "muriatic acid" / lab concentrated HCl.
  • 98% w/w concentrated H₂SO₄: typical lab concentrated sulfuric.
  • 70% w/w isopropanol: standard surface disinfectant (the 30% water gives the alcohol time to penetrate cell membranes).
  • 24K gold = 100% Au; 18K = 75%; 14K = 58.3%; 10K = 41.7%.
  • Sterling silver = 92.5% Ag, 7.5% Cu (the residual copper hardens the alloy).
  • Stainless steel 304: 18-20% Cr, 8-10.5% Ni, balance Fe.
  • Earth crust: 46.6% O, 27.7% Si, 8.1% Al, 5.0% Fe (by mass).
  • Human body: ~65% O, ~18% C, ~10% H, ~3% N, ~1.5% Ca (by mass).

Mass Percent vs Other Concentration Units

  • % w/w (mass percent): mass solute / mass solution × 100. Temperature-independent.
  • % w/v (mass-volume percent): g solute per 100 mL of SOLUTION. Common in clinical and pharma; depends on density (≈ % w/w only for unit-density aqueous solutions).
  • % v/v (volume percent): mL solute per 100 mL solution. Standard for liquor (ABV) and liquid-in-liquid mixtures.
  • Mass concentration (g/L): mass per liter of solution. For dilute aqueous: 1% w/w ≈ 10 g/L.
  • Molarity (M, mol/L): moles of solute per liter solution. M = (mass concentration in g/L) / MW.
  • Mole fraction: moles solute / total moles. Used in colligative-property calculations.
  • Molality (m, mol/kg solvent): moles solute per kg of SOLVENT (not solution). Temperature-independent like % w/w.
  • ppm (parts per million): 1 ppm = 1 mg/kg = 0.0001% w/w (in dilute aqueous: 1 ppm ≈ 1 mg/L ≈ 1 µg/g).
  • ppb (parts per billion): 1 ppb = 1 µg/kg = 10⁻⁷ % w/w.
Real-World Example

Worked Example — Saline, Brass, and Glucose Composition

Solute mode — make 0.9% saline. You weigh 4.5 g of NaCl and dissolve in 495.5 g of water.

  • Mass of solution = 4.5 + 495.5 = 500 g.
  • % w/w = (4.5 / 500) × 100 = 0.9% ✓ (matches physiological saline).
  • Equivalent: 9000 ppm or 9,000,000 ppb.

Chemical mode — gold karat. An 18-karat gold ring weighs 8.0 g and contains 6.0 g of pure gold.

  • % Au = (6.0 / 8.0) × 100 = 75% w/w (matches the 18K = 18/24 = 75% definition).
  • Remaining 25% is copper, silver, or other alloying metals.

Composition mode — sodium chloride NaCl. Pick Na (1 atom) + Cl (1 atom).

  • Na: 1 × 22.990 = 22.990 g/mol.
  • Cl: 1 × 35.45 = 35.45 g/mol.
  • Molar mass = 58.44 g/mol.
  • % Na = (22.990 / 58.44) × 100 = 39.34%.
  • % Cl = (35.45 / 58.44) × 100 = 60.66%.
  • This is why a 1 g NaCl tablet provides ~393 mg sodium — relevant for low-sodium dietary calculations.

Composition mode — calcium carbonate CaCO₃. Pick Ca (1) + C (1) + O (3).

  • Ca: 1 × 40.078 = 40.078; C: 1 × 12.011 = 12.011; O: 3 × 15.999 = 47.997.
  • Molar mass = 100.086 g/mol.
  • % Ca = 40.04%, % C = 12.00%, % O = 47.96%.
  • Calcium-supplement labeling: 1250 mg CaCO₃ = 500 mg elemental calcium (40% of the tablet by mass) — a common confusion on supplement labels.

Who Should Use the Mass Percent Calculator?

1
Make solutions of specified % w/w concentration — saline, sucrose, buffer salts. The calculator gives the exact mass of solute and solvent for any target percentage and total mass.
2
Convert combustion-analysis percentages to empirical formulas (assume 100 g sample, divide each percentage by atomic weight, ratio gives formula). Verify proposed formulas against measured % composition.
3
Compute % active ingredient in tablets, syrups, creams. A 500 mg ibuprofen tablet weighing 750 g (with binders/excipients) is 66.7% w/w active.
4
Calculate % macronutrients on food labels — fat, protein, carbohydrate as % of total weight. Convert serving-size grams to percentages.
5
Grade alloys by composition — brass, bronze, steel grades, gold karats. The calculator works directly with mass fractions or computes from elemental atom counts.
6
Compute ore grade (% metal in ore) for cost-of-extraction calculations. A 0.5% Cu ore deposit is economic; 0.05% may not be.
7
Convert between % and ppm/ppb for trace contaminants — drinking-water lead 10 ppb max, EPA arsenic 10 ppb, soil lead 400 ppm action level.

Technical Reference

Definitions and SI Conventions. Per IUPAC: mass fraction w of component i = m_i / Σm_j (dimensionless, 0 ≤ w ≤ 1). Mass percent = w × 100. The phrase "percent by weight" is colloquially used but technically incorrect — weight is a force; mass is what we measure on a balance. Both terms refer to mass percent in practice.

The Three Forms. (1) % w/w (mass-mass percent): mass solute / mass solution × 100. Dimensionless; temperature-independent. Standard for bulk chemistry, food labels, alloys. (2) % w/v (mass-volume percent): g solute per 100 mL of solution. Has units (g/100 mL); depends on density and temperature. Standard in clinical chemistry (D5W = 5% w/v dextrose, 0.9% w/v saline). For dilute aqueous: % w/v ≈ % w/w (within ~1%). (3) % v/v (volume-volume percent): mL solute per 100 mL solution. Standard for liquor (ABV — alcohol by volume), perfume composition, mixed solvents.

Conversion Identities.

  • % w/w → mass concentration (g/L): for dilute aqueous (density ≈ 1 g/mL): % × 10 = g/L. For non-aqueous or dense solutions: g/L = % × density_g_per_mL × 10.
  • % w/w → molarity (mol/L): M = (% × density_g_per_mL × 10) / MW. For 37% HCl (density 1.19, MW 36.46): M = 37 × 1.19 × 10 / 36.46 = 12.07 M.
  • % w/w → ppm: ppm = % × 10,000.
  • % w/w → ppb: ppb = % × 10⁷.
  • % w/w → mole fraction x: x = (w/MW_solute) / (w/MW_solute + (1-w)/MW_solvent). For dilute aqueous (water MW 18.015): x ≈ (w / MW_solute) × 18.015 (when w is small).

Atomic Weights (IUPAC 2021). The calculator uses the conventional atomic weights from the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW) 2021 recommendations. For elements with naturally varying isotopic compositions (H, Li, B, C, N, O, Mg, Si, S, Cl, Br, Tl), CIAAW reports an INTERVAL — the calculator uses the midpoint of the interval (e.g. C = 12.011, H = 1.008, O = 15.999). Variation across natural samples is small (typically ±0.001 g/mol for common elements) and below the precision needed for most calculations. For NIST-traceable analytical work, use the specific atomic weight of your reference standard's isotopic composition.

Empirical Formula from Combustion Analysis. Combustion analysis measures % C, % H, % N (and oxygen by difference). To go from percentages to empirical formula: (1) assume 100 g sample → percentages become grams; (2) divide each gram-value by the element's atomic weight to get moles; (3) divide all mole values by the smallest to get whole-number ratios; (4) multiply if needed to clear fractions. Example: a sample is 40.0% C, 6.71% H, 53.3% O. Moles per 100 g: C = 40.0/12.011 = 3.33; H = 6.71/1.008 = 6.66; O = 53.3/15.999 = 3.33. Ratio C:H:O = 1:2:1 → empirical formula CH₂O. Combined with molar mass (e.g. by mass spectrometry: 180 g/mol) gives molecular formula = (CH₂O)₆ = C₆H₁₂O₆ (glucose).

Hydrates and Crystal Water. Many salts crystallize with stoichiometric water of hydration that is part of the formula mass. For mass-percent calculations involving hydrates: (1) use the hydrate formula and molar mass for percent composition (e.g. CuSO₄·5H₂O has 36.07% water by mass); (2) for solution prep at a target molarity, use the hydrate molar mass (249.69) when weighing the hydrate; (3) for "anhydrous equivalent" calculations, multiply hydrate mass by (anhydrous MW / hydrate MW). A 1 g sample of CuSO₄·5H₂O contains only 159.61/249.69 = 0.639 g of anhydrous CuSO₄ and 0.361 g of water.

Karat Gold and Sterling Silver Conventions. Karat (K or kt) is a 24-part fineness scale: 24K = 100% gold by mass; 22K = 91.67%; 18K = 75%; 14K = 58.33%; 10K = 41.67%. Below 10K is not legally "gold" in most jurisdictions (US, UK). For silver: fine silver = 99.9%; sterling = 92.5%; coin silver = 90%; Britannia = 95.8%. The remainder is typically copper (which hardens the alloy). Hallmarking laws require these percentages on jewelry. For platinum, the convention is parts per thousand: 950 platinum = 95.0% w/w.

Conclusion

Mass percent is the bedrock concentration unit — temperature-stable, dimensionless, and equally applicable to solutions, mixtures, alloys, and pure compounds. Master the three forms (solute in solution, chemical in mixture, percent composition by element) and you cover ~90% of routine concentration calculations in chemistry, pharmacology, food science, and metallurgy.

The two failure modes to watch for: (1) wrong denominator — dividing by mass of solvent instead of mass of solution gives inflated values for dilute mixtures; (2) hydrate confusion — using anhydrous molar mass for a hydrated salt (or vice versa) gives 30-50% errors. The calculator handles both by enforcing m_solution = m_solute + m_solvent and providing a built-in periodic table for percent composition. Use ppm/ppb for trace work, % for bulk, and always state which mode (w/w vs w/v vs v/v) you are using — the labels look similar but can differ by 5-15% for dense solutes.

Frequently Asked Questions

What is the Mass Percent Calculator?
It implements the foundational identity % w/w = (mass of part / total mass) × 100 in three modes: (1) Solute mode — enter any 2 of 3 mass fields (solute, solvent, solution) and the calculator solves for the third using m_solution = m_solute + m_solvent. (2) Chemical mode — direct mass-fraction calculation for any chemical inside any mixture (alloys, ores, drug content, food labels). (3) Percent composition mode — pick up to 6 elements with atom counts and the calculator computes molar mass and the mass percent of every element using IUPAC 2021 atomic weights. Outputs in % w/w, fraction, ppm, and ppb simultaneously.

Pro Tip: Pair this with our Molarity Calculator for solution preparation.

What is the formula for mass percent?
% w/w = (mass of solute / mass of solution) × 100, where mass of solution = solute + solvent (NOT just solvent — a frequent error). Equivalently as a fraction: w = m_solute / m_solution, with 0 ≤ w ≤ 1. To convert: % w/w = w × 100. For percent composition by element: %_element = (count × atomic_weight / molar_mass) × 100, where molar_mass = Σ(count × atomic_weight) over all elements in the formula.
What is the difference between % w/w, % w/v, and % v/v?
% w/w (mass-mass): grams of solute per 100 g of solution; dimensionless and temperature-independent. % w/v (mass-volume): grams of solute per 100 mL of solution; has units (g/100 mL); depends on density and temperature. Standard for clinical (D5W = 5% w/v dextrose, 0.9% w/v saline). For dilute aqueous solutions, % w/v ≈ % w/w within ~1%. % v/v (volume-volume): mL of solute per 100 mL of solution; standard for liquor (ABV) and liquid-in-liquid mixes (ethanol/water, perfume composition). Common error: a label that says "5% NaCl" without specifying w/w vs w/v leaves ~5% ambiguity in the actual concentration.
How do I convert mass percent to ppm or ppb?
ppm = % w/w × 10,000. ppb = % w/w × 10⁷. Examples: 1% = 10,000 ppm = 10⁷ ppb. 0.1% = 1,000 ppm = 10⁶ ppb. 0.001% = 10 ppm = 10,000 ppb. EPA drinking-water lead limit is 15 ppb = 1.5 × 10⁻⁶ % w/w — using % directly for trace contaminants would be 0.0000015%, which is unreadable. Use ppm for trace species in food, water, soil; use ppb for ultra-trace contaminants and biomarkers. The calculator displays all four units simultaneously.
How do I find percent composition from a molecular formula?
%_element = (count × atomic_weight / molar_mass) × 100. Workflow: (1) count the atoms of each element in the formula; (2) multiply each count by the element's atomic weight (sum gives molar mass); (3) for each element, divide its mass contribution by the molar mass and multiply by 100. Example for glucose C₆H₁₂O₆: C = 6 × 12.011 = 72.066; H = 12 × 1.008 = 12.096; O = 6 × 15.999 = 95.994; molar mass = 180.156 g/mol. % C = 72.066/180.156 × 100 = 40.00%; % H = 6.71%; % O = 53.29%. Sum = 100%. Use Composition mode in the calculator for an instant answer.
What does "100 g sample assumption" mean for empirical formulas?
When converting % composition into an empirical formula, assume the sample weighs exactly 100 g — then each % becomes grams directly. Workflow: (1) percentages → grams (in 100 g basis); (2) divide each gram value by the atomic weight to get moles; (3) divide all mole values by the smallest to get integer ratios. Example: 40.0% C, 6.71% H, 53.3% O → 100 g basis: 40.0 g C, 6.71 g H, 53.3 g O → moles: 3.33, 6.66, 3.33 → ratios: 1, 2, 1 → empirical formula CH₂O. With molar mass from MS (180 g/mol): molecular formula = (CH₂O) × 6 = C₆H₁₂O₆ (glucose).
Why is mass percent independent of temperature but molarity is not?
Mass percent has no volume term — it's pure mass / pure mass — and mass does not change with temperature (mass is conserved across thermal expansion). Molarity has a volume term (mol/L) — and the liter changes with temperature because liquids expand. A 1.000 M solution at 25 °C becomes ~0.998 M at 35 °C and ~0.978 M at 80 °C from water expansion alone. For temperature-stable concentration reporting in food, pharma, and bulk chemistry, % w/w is preferred. For benchwork at controlled lab temperature, molarity is fine. For colligative-property work (boiling-point elevation, osmotic pressure), use molality (mol/kg solvent) — also temperature-independent.
What is the mass percent of water in CuSO₄·5H₂O?
36.07%. Math: 5 H₂O = 5 × 18.015 = 90.075 g/mol; CuSO₄·5H₂O total = 249.69 g/mol; %_water = 90.075 / 249.69 × 100 = 36.07%. Practical implication: when you weigh out 1 g of the blue pentahydrate, you are getting only 0.639 g of anhydrous CuSO₄ — the other 0.361 g is crystal water that contributes no Cu²⁺ or SO₄²⁻ ions. Hydrate confusion is one of the most common sources of 30-50% concentration error in undergraduate chemistry. Always check the bottle label for "·nH₂O" notation; "anhydrous" or "(A)" indicates the water-free form.
What is karat gold in mass percent?
Karat (K) is a 24-part fineness scale. 24K = 100% gold (pure, soft, used for bullion and some jewelry). 22K = 22/24 = 91.67% (high-end jewelry, Asian markets). 18K = 75% (premium jewelry — strong, retains color). 14K = 58.33% (US standard for engagement rings). 10K = 41.67% (legal minimum for "gold" in US/UK). The remaining percentage is alloying metals — typically copper (gives rose tint), silver (gives green/white tint), nickel or palladium (white gold). For sterling silver: 92.5% Ag (rest is copper). For platinum: 950 platinum = 95.0% Pt by mass; the convention is parts per thousand, not karats.
How do I compute % NaCl in a saline solution?
% NaCl (w/w) = (mass NaCl / mass solution) × 100, where mass solution = mass NaCl + mass water. Example: dissolve 9 g NaCl in 991 g water → mass solution = 1000 g → % w/w = 0.9%. This is physiological saline (also called normal saline or 0.9% NS) — isotonic with human plasma (~290 mOsm/L). 3% NaCl = hypertonic saline (used for severe hyponatremia in ICU). 0.45% NaCl = half-normal saline (hypotonic, used for hypernatremia correction). To convert to molarity: M = (% × 10) / 58.44 = 0.9 × 10 / 58.44 = 0.154 M (close enough to 0.15 M physiological).
How do I read a food nutrition label using mass percent?
Food labels report grams per serving — convert to % by dividing by total serving weight. Example: a 30 g cheese serving with 9 g fat, 7 g protein, 1 g carbs has: % fat = 9/30 = 30%; % protein = 7/30 = 23.3%; % carbs = 1/30 = 3.3%; % water + ash ≈ 43%. The Daily Value (DV) percentages on US labels are different — they are % of recommended daily intake, not % of food mass. Macronutrient calorie distribution: calories = 9 × g_fat + 4 × (g_protein + g_carbs); for the cheese above: 9×9 + 4×(7+1) = 81 + 32 = 113 kcal — useful for diet planning even though calorie % ≠ mass %.

Author Spotlight

The ToolsACE Team - ToolsACE.io Team

The ToolsACE Team

Our ToolsACE chemistry team built this calculator to handle the three distinct interpretations of <strong>mass percent</strong> in a single tool. <strong>(1) Solute mode</strong> — enter any 2 of 3 fields (mass of solute, mass of solvent, mass of solution) and the calculator solves for the third using the constraint <strong>m_solution = m_solute + m_solvent</strong>, then reports % w/w. <strong>(2) Chemical mode</strong> — enter mass of chemical and total mass of mixture for direct mass-fraction calculations on alloys, ores, drug content, food labels, and purity assays. <strong>(3) Percent composition mode</strong> — pick up to 6 elements with atom counts (e.g. glucose = 6 C, 12 H, 6 O); the calculator computes molar mass = Σ(count × atomic weight) and the mass percent of every element. All masses accept g / mg / µg / kg / lb / oz; all results expressed simultaneously as % w/w, fraction, ppm, and ppb. Built-in IUPAC 2021 atomic weights for 37 of the most common elements.

IUPAC 2021 standard atomic weightsStandard analytical chemistry referencesCRC Handbook of Chemistry and Physics

Disclaimer

Mass percent (% w/w) requires the WHOLE in the denominator — solute + solvent for solutions, total sample mass for mixtures. Dividing by mass of solvent only inflates dilute values. Mass percent is distinct from % w/v (g per 100 mL of solution; depends on density) and % v/v (mL per 100 mL of solution); the labels look similar but values can differ by 5-15% for dense solutes. Hydrate forms (CuSO₄·5H₂O = 36.07% water by mass) are a frequent error source — always check the form on the supplier's Certificate of Analysis. Atomic weights are IUPAC 2021 conventional values; isotopic variation in natural samples is typically <0.01% for common elements. References: IUPAC Compendium of Chemical Terminology, CIAAW 2021 atomic weights, CRC Handbook of Chemistry and Physics.