How to Work Chemistry Problems
Many real-world chemistry problems involve determining the amount of one substance in a chemical equation given the amount of another. To do this, you need to convert from molecular formulas to physical quantities such as mass and volume. The bridge between these two values is the "mole." A mole of a solid or liquid substance is 6.022 x 10^23---Avogadro's number---of atoms or molecules and is equal to the molecular weight in grams. For gas volume, a mole of atoms or molecules is 22.4 L at standard temperature and pressure (STP).
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Instructions
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1
Write out the problem statement and determine the given and needed information. For example:
Problem: How much carbon dioxide---CO2---is needed to produce 10 grams of calcium carbonate---CaCO3---from a calcium hydroxide---Ca(OH)2---solution?
Given: 10g CaCO3
Needed: volume of CO2 at STP -
2
Write the balanced chemical equation for the reaction. Start by writing the skeleton equation and then balance the number of atoms for each element on each side of the equation. In this example, the skeleton equation is already balanced. The extra water molecule that forms becomes a part of the Ca(OH)2 solution and the CaCO3 precipitates out.
CO2 + Ca(OH)2 ---> CaCO3 + H2O
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3
Convert the given quantity from grams to moles. For this example, add the atomic weights, which are written on most periodic tables, for each element in CaCO3 to determine the molecular weight and then do the conversion.
Molecular weight: 40.08 + 12.01+ 3 (16.00) = 100.09
Conversion: 10g * (1 mole/100.09 g) = 0.01 moles -
4
Calculate the mole ratio for the given and unknown substances. The mole ratio is the coefficient of unknown quantity divided by the coefficient of the known value. In this example, the coefficients for both in the chemical equation are one, so the mole ratio is one. For the generation of ammonia, N2 + 3H2 ---> 2NH3, the mole ratio for ammonia to hydrogen is 2/3.
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Multiply the number of moles of the known quantity by the mole ratio to get the molar quantity of the unknown substance. Multiply the molar quantity by the mole conversion for the unknown quantity to get the result.
0.01 moles CaCO3 * 1 (mole ratio) = 0.01 moles CO2
0.01 moles CO2 * (22.4 L / 1 mole) = 2.24L CO2 at STP
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