Unit 1 · Atomic Structure and Properties

AP Chemistry Molar Mass

Calculating molar mass from any formula, including parentheses and hydrates, then using it for percent composition, in four worked examples.

Molar mass is the first step of almost every quantitative chemistry problem: converting grams to moles, finding a molarity, or working out a theoretical yield. The calculation is only addition, but errors in reading the formula cost points on questions that are otherwise easy. This page shows the method, a table of common values, and the cases where students slip: parentheses, hydrates, and percent composition. The next step, converting between grams, moles, and particles, is in mole conversions.

How to Calculate Molar Mass From a Chemical Formula

1. Count the atoms of each element. A subscript applies to the element or group just before it, and a subscript after parentheses multiplies everything inside.
2. Look up each atomic mass on the periodic table. The AP periodic table prints the atomic mass of every element, so no values need to be memorized.
3. Multiply and add: molar mass = Σ(number of atoms × atomic mass), reported in g/mol.

The bars show where the mass comes from in two formulas, drawn to the same scale: the subscripts decide how large each element's share is. This page rounds atomic masses to common two-decimal values, so a result can differ from your own in the last digit depending on the table you use. Keep the significant figures consistent with the masses given.

To check a formula, the molar mass calculator shows each element’s share and the percent composition, and converts grams, moles, and particles.

Molar Masses of Common Substances

These use H 1.008, C 12.01, N 14.01, O 16.00, Na 22.99, S 32.06, Cl 35.45, and Ca 40.08. On the exam, use the values printed on your periodic table; the aim is the method, not the memorized number.

Worked Example 1: Finding the Molar Mass of Ethanol, Ammonia, and Calcium Carbonate

Calculate the molar mass of C₂H₅OH, NH₃, and CaCO₃.

C₂H₅OH has 2 C, 6 H (five in the C₂H₅ group plus one in OH), and 1 O: 2(12.01) + 6(1.008) + 16.00 = 24.02 + 6.048 + 16.00 = 46.07 g/mol.
NH₃: 14.01 + 3(1.008) = 14.01 + 3.024 = 17.03 g/mol.
CaCO₃: 40.08 + 12.01 + 3(16.00) = 40.08 + 12.01 + 48.00 = 100.09 g/mol.

In ethanol, the hydrogens are written in two places, so count all of them. Writing the formula as C₂H₆O gives the same total and is a good way to check.

Worked Example 2: Handling Parentheses in Calcium Nitrate and Iron(III) Sulfate

Calculate the molar mass of Ca(NO₃)₂ and Fe₂(SO₄)₃.

Ca(NO₃)₂: the 2 multiplies everything inside the parentheses, giving 1 Ca, 2 N, and 6 O.
40.08 + 2(14.01) + 6(16.00) = 40.08 + 28.02 + 96.00 = 164.10 g/mol.
Fe₂(SO₄)₃: 2 Fe, 3 S, and 3 × 4 = 12 O.
2(55.85) + 3(32.06) + 12(16.00) = 111.70 + 96.18 + 192.00 = 399.88 g/mol.

The polyatomic ion in the parentheses is a unit, and the subscript after it multiplies each of its atoms. Knowing the ion formulas, such as nitrate NO₃⁻ and sulfate SO₄²⁻, makes these formulas easy to read; they are listed on the polyatomic ions page.

Worked Example 3: Including the Water in a Hydrate

Find the molar mass of copper(II) sulfate pentahydrate, CuSO₄·5H₂O, and the percent of its mass that is water.

CuSO₄: 63.55 + 32.06 + 4(16.00) = 159.61 g/mol.
5H₂O: 5 × 18.016 = 90.08 g/mol.
Hydrate: 159.61 + 90.08 = 249.69 g/mol.
Percent water: (90.08 ÷ 249.69) × 100 = 36.08%.

The dot means five water molecules are part of each formula unit, so they count toward the molar mass. Heating a hydrate drives off exactly this 36.08% of its mass, which is the basis of the lab question that finds the number of waters from a mass loss.

How an incomplete heating or a damp sample changes the calculated percent water, and how to report the percent error, is worked through in AP Chemistry percent error.

Worked Example 4: Turning Molar Mass Into Percent Composition by Mass

Find the percent by mass of each element in glucose, C₆H₁₂O₆ (molar mass 180.16 g/mol).

Carbon: 72.06 ÷ 180.16 × 100 = 40.00%.
Hydrogen: 12.10 ÷ 180.16 × 100 = 6.71%.
Oxygen: 96.00 ÷ 180.16 × 100 = 53.29%.
Check: 40.00 + 6.71 + 53.29 = 100.00%.

Each percentage is the element's total mass in one mole divided by the molar mass. The calculation can be run backward to find a formula from percentages, which is the method in empirical and molecular formulas.

Five Molar Mass Practice Questions With Answers

1. Find the molar mass of Mg(OH)₂.
   Show answer

   24.31 + 2(16.00 + 1.008) = 24.31 + 34.02 = 58.33 g/mol.
2. Find the molar mass of (NH₄)₂SO₄.
   Show answer

   2 N = 28.02, 8 H = 8.064, 1 S = 32.06, 4 O = 64.00, for a total of 132.14 g/mol.
3. What percent of the mass of CO₂ is oxygen?
   Show answer

   32.00 ÷ 44.01 × 100 = 72.71%.
4. Find the molar mass of Na₂CO₃·10H₂O and its percent water.
   Show answer

   Na₂CO₃ = 45.98 + 12.01 + 48.00 = 105.99 g/mol, and 10H₂O = 180.16 g/mol, so the hydrate is 286.15 g/mol. Percent water = 180.16 ÷ 286.15 × 100 = 62.96%.
5. A student finds the molar mass of Ca(OH)₂ as 40.08 + 16.00 + 1.008 = 57.09 g/mol. What went wrong?
   Show answer

   The subscript 2 applies to the whole OH group, so there are two O and two H. The correct value is 40.08 + 2(16.00) + 2(1.008) = 74.10 g/mol.

Common Molar Mass Mistakes

• Ignoring the subscript after parentheses. It multiplies every atom inside.
• Leaving out the waters in a hydrate. The dot and the number before H₂O count toward the molar mass.
• Using the atomic number instead of the atomic mass. The mass is the decimal number on the periodic table.
• Rounding atomic masses too early. Add the full values first, then round the total.
• Missing hydrogens in a condensed formula. C₂H₅OH has six hydrogens, not five.
• Writing the wrong units. Molar mass is in g/mol; atomic mass units are for single particles.

A correct molar mass makes the rest of a stoichiometry problem straightforward. To see how a stronger Unit 1 would shift your overall estimate, the AP Chem Score Calculator turns section scores into a 1–5 range.

Related Resources

• AP Chem Score Calculator
• Unit 1 Review: Atomic Structure and Properties
• AP Chemistry Mole Conversions
• Molar Mass Calculator
• Percent Yield Calculator
• AP Chemistry Avogadro's Number
• AP Chemistry Average Atomic Mass
• AP Chemistry Empirical & Molecular Formula
• AP Chemistry Polyatomic Ions
• AP Chemistry Significant Figures Rules
• AP Chemistry Molarity
• AP Chemistry Density
• AP Chemistry Study Guide

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