Mass Percent Calculator

Mass percent (or weight percent) is the mass of a component divided by the total mass of the mixture, multiplied by 100. It expresses what percentage of a compound's mass comes from each element. For example, water (H₂O) is 11.19% hydrogen and 88.81% oxygen by mass because: mass of H = 2 × 1.008 = 2.016g, mass of O = 16.00g, total = 18.016g, %H = 2.016/18.016 × 100 = 11.19%.

Step 1: Write the formula and parse into elements with counts (e.g., CO₂ → C:1, O:2). Step 2: Find atomic masses from periodic table. Step 3: Calculate molar mass: add (atomic mass × count) for each element. Step 4: Calculate mass of target element in one mole. Step 5: Divide element mass by molar mass and multiply by 100. For CO₂: M = 44.01 g/mol, mass O = 32.00 g, %O = 32.00/44.01 × 100 = 72.73%.

Mass percent relates to mass (grams) while mole percent relates to number of molecules (moles). They differ because atoms of different elements have different masses. For NaCl: 39.34% Na and 60.66% Cl by mass (based on 22.99g and 35.45g). By moles, it's exactly 50% Na and 50% Cl because 1:1 ratio. For CO₂: 27.29% C by mass, but only 33.3% C by moles (1 C out of 3 atoms).

Step 1: Convert mass percent to grams (assume 100g sample). Step 2: Convert grams to moles by dividing by atomic mass for each element. Step 3: Divide all mole values by the smallest mole to get simplest ratio. Step 4: If ratios aren't whole numbers, multiply by smallest factor to get integers. Example: 40% C, 6.67% H, 53.33% O → 3.33 mol C, 6.67 mol H, 3.33 mol O → CH₂O (empirical formula of glucose).

No, mass percent cannot exceed 100% because it represents a portion of the whole. If you calculate a mass percent greater than 100%, there's an error in your calculation or data. Common errors include: using atomic mass of compound instead of element, forgetting to divide by total molar mass, double-counting mass, or measurement errors. Always verify that mass percents of all elements in a compound sum to 100%.

Mass percent is used in: (1) Quality control - verifying compound purity, (2) Solution preparation - calculating solute mass for desired concentration, (3) Nutritional labels - showing percent of daily value, (4) Alloy composition - specifying metal percentages in mixtures, (5) Environmental analysis - pollutant concentrations in soil/water, (6) Manufacturing - formulation of paints, cements, and pharmaceuticals.