I've got a two part question below with my answer and the mark scheme answer below, I'd like it if somebody could explain the mark scheme answers as they've confused me.
Question 1 A volumetric flask contains a very precise amount of water. The amount the flask contains is 500 ml of water.
a) A student wants to make up a solution of sodium carbonate of concentration 0.1 mol/dm
3. What mass of sodium carbonate needs to be added to the flask?
My answer to part a)Moles = mass X concentration
Moles = 0.1 X 0.5 = 0.05 moles of sodium carbonate
Mass = moles X Mr
Mass = 0.05 X 106 = 5.3 grams of sodium carbonate needed.
Mark scheme answer to part a)1 mole of sodium carbonate weighs 106 grams
106/10 = 10.6
10.6/2 = 5.3 grams of sodium carbonate needed.
Question 1 part bIf a sample of 10 cm
3 is taken from the flask, how many moles of sodium carbonate will be in the sample?
My answer to part b)Moles = concentration X volume
We know the concentration of sodium carbonate is 0.1 mol/dm
3Moles = 0.1 X 0.01 = 0.001 moles of sodium carbonate in the sample.
Mark scheme answer to part b)Mass of sodium carbonate = 5.3 grams
Moles of sodium carbonate = 0.05 moles
Now divide moles by 50 so that 0.05/50 = 0.001 moles of sodium carbonate in the sample.
So my answers are right, however I've reached them in a different way from the mark scheme, can someone explain the mark scheme answers to me as it's left me totally confused