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Mass-Mole Calculations (n = mass ÷ MM)

Key Concepts

  • 1 mole of a pure substance has a mass equal to its molecular mass (MM)
        So 2 moles would have a mass = 2 x MM
        3 moles would have a mass = 3 x MM etc

  • This leads to the formula: mass = n x MM
        mass is in grams,
        n = moles of pure substance,
        MM = molecular mass of the pure substance

  • This formula can be rearranged to give the following:
        n = mass ÷ MM
        MM = mass ÷ n

Examples

1. mass = n x MM

Calculate the mass of 0.25 moles of water

  • mass (in grams) = n x MM

  • n = 0.25mol

  • MM of water (H2O) = (2 x 1.008) + 16.00 = 18.016g mol-1

  • mass = 0.25 x 18.016 = 4.504g

2. n = mass ÷ MM

Calculate the moles in 124.5g of oxygen gas.

  • n = mass ÷ MM

  • mass = 124.5g

  • MM of O2 = 2 x 16.00 =32.00g mol-1

  • n = 124.5 ÷ 32.00 = 3.89mol

3. MM = mass ÷ n

Calculate the molecular mass of a pure substance if 1.75 moles of the substance has a mass of 29.792g

  • MM = mass ÷ n

  • mass = 29.792g

  • n = 1.75mol

  • MM = 29.792 ÷ 1.75 = 17.024g mol-1
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Related AUS-e-TUTE Topics

Interactive Periodic Table of the Elements

Naming Ionic Compounds

Writing Ionic Formula

Calculating Molecular Mass

Calculating Percentage (Percent) Composition

Definitions of a Mole

Calculating Molarity of Solutions

Dilution Calculations

 

Practice

To calculate the mass given the moles and molecular mass, enter the moles of compound and its molecular mass, then click the Calculate button.
Enter moles: mol
Enter the molecular mass: g/mol
Click Calculate:
Calculation:
Mass = g
To start again, Click Reset:

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