i have done the maths based on an online conversion calculator that said 1 liter of air is equal to 0.001m^3 in volume. my chemistry teacher tells me that my maths is wrong but wont tell me how... here is what i wrote: Failing the above, a larger crucible would hold more oxygen and the magnesium would be able to burn for longer before the lid would have to be lifted. This wouldn’t prevent the error given by lifting the crucible lid but it would minimise it to best practice. It would be possible to measure how large the crucible would need to be by working out the molecular weight of oxygen in a given area of air (air is roughly 21% oxygen). To react the magnesium with enough oxygen the same amount of each element is needed to give the 1:1 ratio meaning if the weight of magnesium given was 24.31 grams we would need 16 grams of oxygen to complete the reaction. 24.31 / 16 = 1.519375. This gives the difference between the weights required. 1.519375 grams of magnesium is necessary if you had 1g of oxygen to react with. The magnesium weight at the beginning of the experiment was 0.26 grams. 24.31 / 0.26 = 93.5. The amount of magnesium given would fit into 1 mole 93.5 times. 16 / 93.5 = 0.171122994. This is the minimal amount of oxygen required in grams for this reaction to have worked completely without additional oxygen having to be added by lifting the crucible lid. Having this number would mean that you could work out how much air would need to be in contact with the magnesium, for there to be enough oxygen available to react with the magnesium without completely using it up and having to lift the lid. For example if we had 21 grams of air inside our crucible, in theory 1 gram of oxygen would be available for this reaction and it would have been enough. If it is said that air is 100% and 21% of it is needed (the oxygen). 100/21 = 4.761904762. This is the amount of times 21 fits into the 100. Now multiplying the amount of oxygen required by this number should give us the total mass of the air needed for our reaction to be successful without having to add additional air. 0.171122994 x 4.761904762 = 0.8148714 grams of air would be required. The density of air is approximately 1.19g/cm3 The crucibles were larger than 1cm3 on the inside so there should have been enough oxygen overall inside to react with the magnesium. please can someone help me figure out where i went wrong?