Q. Find the percentage a mass of magnesium in magnesium oxide MgO given the relative atomic mass of magnesium is twenty-four and that of oxygen is sixteen.


A. Using the equation % mass Mg = (24 x 1/40) x 100 = 60%

 

Don't forget to use a number of atoms of the element in the calculation, even if it happens to be one, because if you don't get used to doing this you may omit it inadvertently in future calculations and get the whole thing wrong.


Q. Calculate the percentage mass of sulphur in sulphuric acid given the formula of sulphuric acid H2SO4 and the relative atomic masses of H = 1, O = 16 and S = 32


A. The relative formula mass of sulphuric acid using the values given is 98 and we can see that there is only one sulphur atom per molecule, so substituting these values into the equation % mass S in H2SO4 = ((32 x 1) / 98) x 100 = 32.7% rounded to one decimal place.


Q. Calculate the percentage by mass of manganese in potassium manganate (VII) (potassium permanganate), given the relative atomic masses of potassium = 39 manganese = 55 and oxygen = 16.


A. The molecular formula for potassium manganate (VII) (potassium permanganate) is KMnO4  and therefore its relative molecular mass would be 39+55+ (4×16) = 158. the percentage mass of manganese is therefore ((55×1) / 158) = 34.8% rounded to one decimal place.


Part of the art of answering chemistry questions, or indeed any science question, is making sure you've actually established what the question wants. Examiners can construct questions in such a way that you need to study it quite carefully to establish what it is exactly asking you well before you set about calculating it. Many students have the knowledge but respond with the wrong answers because they have clearly misunderstood the question.


Q. A mixture of iron oxide and sodium chloride solids contains 36% sodium ions by mass. Given that the only compound containing sodium is the sodium chloride, calculate the mass of sodium chloride in 120 g of the mixture.


A. First of all let us work out how much sodium there is in the mixture by mass. if we have 120 g of the mixture and 36% of the total mass is sodium then we will have (36÷100) x 120 = 43.2g. The relative formula mass of sodium chloride is 58.5 and the percentage mass of sodium in sodium chloride is (23/58.5) x 100 = 39.3%. We are told at the beginning of the question that we have a 120 g of the mixture and at 43.2 g of it is sodium,then the mass of sodium chloride in the mixture is 43.2 x (100/39.3) = 110g


Q. A mixture contains 40% magnesium by mass. The only compound in the mixture that contains magnesium is magnesium oxide MgO. Calculate the mass of magnesium oxide in 45 g of the mixture.


A. First of all calculate the amount of magnesium by mass in the whole mixture, this is simply (40/100) * 45 = 18g. So now we know that in the total mixture there is 18 g of magnesium. The percentage of magnesium in magnesium oxide is now calculated as (24 / 40) * 100 = 60%. The mass of magnesium oxide in 45 g to give us 18 g of magnesium is therefore 18÷0.6 = 30g


What we will do now is step through another one of these calculations, but literally spell out each step to make sure that you are confident that you can attack this type of problem.


Q.A mixture of lead nitrate (solid) and potassium chloride (solid) contains 6% nitrogen atoms by mass. Given that the only substance in the mixture containing nitrogen is lead nitrate, calculate the mass in grams of lead nitrate contained in 225 g of the mixture.


A.This is probably about as nasty as they could get, but one of my favourite expressions is "never say never" so here we go.......


1. First of all, given the percentage of N in the 225g mix, work out the number of grams that this represents:


6/100 x 225 = 13.5g


2. The percentage of nitrogen in lead nitrate is calculated.......... (2 x 14) / (331) x 100 = 8.5%


3. We therefore have 13.5 g of nitrogen in 225 g of the mixture which is 8.5% rich, therefore the amount of the mixture which contains lead nitrate is (13.5÷0.085) = 158g


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