Calculations From Equations - Chemical Ideas 1.3,1.4 and 4.1
In chemical equations, an important thing to remember is that the number in front of the formulae in the amount of moles involved. For the example below there are 2 moles of oxygen needed to produce the 1 mole of carbon dioxide and 2 moles of water.
CH4 + 2O2 ---------> CO2 + 2H2O
Working out masses
To calculate the mass of 1 mole of an element/compound (Mr) add the relative atomic masses (Ar) of the atoms involved. Calculating masses can be done by following the simple steps below.
Step 1) Write balanced equation out
Step 2) Work out formula masses for the relevant substances
Step 3) Convert the masses to the ones given in the question
Step 4) Convert the other mass in the same way
Example: Work out how many grams of sodium chloride are produced when 5.3g of sodium carbonate reacts with dilute hydrochloric acid.
Step 1) Na2CO3 + 2HCl -----> 2NaCl + H2O + CO2
Step 2) Na2CO3 = 1x (23+23+12+16+16+16) =106
2NaCl = 2x (23+35.5) =117
Step 3) Na2CO3 = (106/106) x 5.3g = 5.3g
Step 4) 2NaCl = (117/106) x 5.3g = 5.85g
Therefore 5.85g of NaCl is produced when 5.3g of Na2CO3 reacts with HCl.
Note: In step 3 and 4 you divide by 106 because this is the Ar of Na2CO3. If there was to be 2 moles of in an equation, such as magnesium the Ar (24.3) would be double and we would divide both Ar of the two substances by 48.6.