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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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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:
 

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

 

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