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Before this: Molar mass

Converting between mass and moles

Balances measure grams, but reactions happen in moles. Molar mass is the bridge between them.

Worked example: grams to moles

How many moles are in 36.0 g of H2O?

Molar mass of H2O = 18.015 g/mol

36.0 g ÷ 18.015 g/mol = 2.00 mol

Worked example: moles to grams

What is the mass of 0.250 mol of NaCl?

Molar mass of NaCl = 22.990 + 35.45 = 58.44 g/mol

0.250 mol × 58.44 g/mol = 14.6 g

Check yourself with units

If the units do not cancel to what you want, the operation is upside down. g ÷ (g/mol) leaves mol; mol × (g/mol) leaves g.

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Watch From grams to moles
  1. A balance reads 36.0 g of water.
  2. One mole of H2O has a mass of 18.015 g.
  3. Divide the mass by the molar mass. Grams cancel: 2.00 mol.

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Explore Grams, moles and particles

Pick a substance and a mass to see the same amount three ways.

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Practice

1. How many moles are in 36.0 g of water, H2O?

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2. What is the mass of 0.250 mol of sodium chloride, NaCl?

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3. How many moles are in 10.0 g of calcium carbonate, CaCO3?

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4. What is the mass of 2.00 mol of CO2?

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5. To convert moles of a substance to grams, you:

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