Molar Calculations

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Calculating masses and gas volumes (notes)

The balanced equation for a reaction shows how many moles of each reactant and product are involved in a chemical reaction.

If the amount, in moles, of one of the reactants or products is known, the number of moles of any other reactants or products can be calculated.

The number of moles (n), the mass of the substance (m), and the molar mass (M) are linked by:

n = m/M 

Note: The molar mass of a substance is the mass per mole of the substance. For CaCO3, for example, the atomic mass of calcium is 40.1, carbon is 12, and oxygen is 16. So the molar mass of CaCO3 is:

40.1 + 12 + (16 x 3) = 100.1 (The units are g mol-1).

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Calculating masses and gas volumes

Look at this worked example; a student heated 2.50g of calcium carbonate, which decomposed as shown inn the equation:

CaCO3 (s) -> CaO (s) + CO2 (g) 

The molar mass of the calcium carbonate is 100.1 g mol-1

a. calculate the amount, in moles, of calcium carbonate decomposes.

n = m/M = 2.50/100.1 = 0.025 mol

b. calculate the amount, in moles, of carbon dioxide that forms.

From the balanced equation, the number of moles of calcium carbonate = number of moles of carbon dioxide = 0.025 mol

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Calculating masses and gas volumes

Look at this worked example; a student heated 2.50g of calcium carbonate, which decomposed as shown inn the equation:

CaCO3 (s) -> CaO (s) + CO2 (g) 

The molar mass of the calcium carbonate is 100.1 g mol-1

a. calculate the amount, in moles, of calcium carbonate decomposes.

n = m/M = 2.50/100.1 = 0.025 mol

b. calculate the amount, in moles, of carbon dioxide that forms.

From the balanced equation, the number of moles of calcium carbonate = number of moles of carbon dioxide = 0.025 mol

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