Stoichiometric calculations Summary Notes Calculations are done using a balanced equation. The calculations involve reacting masses, volume of gases, concentrations of solutions and volumes of solutions. From the equation for a reaction you can tell: how many moles of each substance take part how many grams of each substance take part. The total mass does not change, during a chemical reaction. Calculating masses from equations These are the steps to follow: 1 Write the balanced equation for the reaction. (It gives moles.) 2 Write down the Ar or Mr for each substance that takes part. 3 Using Ar or Mr, change the moles in the equation to grams. 4 Once you know the theoretical masses from the equation, you can then find any actual mass. Calculating gas volumes from equations The concentration of a solution is the amount of solute, in grams or moles, that is dissolved in 1 dm3 of solution. Finding the amount of solute in a solution If you know the concentration of a solution, and its volume: you can work out how much solute it contains, in moles. Just rearrange the equation above: amount of solute (mol) = concentration (mol / dm3) x volume (dm3) you can then convert moles to grams, by multiplying the number of moles by Mr. Calculating concentration from an equation