1a) I think you will have got a couple of marks for that for saying that it is the Mg ions which harden the water, and for saying you get a precipitate of magnesium carbonate.
1c) this was only a 1 mark answer, and because you mentioned water evaporated at 100C, I think you'll get the mark.
1d) you'll get a mark for describing the trend of the graph, and the trend of what should be expected. To get the other marks I think you may have had to quote numbers on the graph.
3b) sorry, I cannot remember the question!
please remind me
4b) this looks ok! You've mentioned the difference in pH and ionisation. I think you may have had to quote an example pH for a strong and a weak acid. But I think that would be a bit picky. I forgot to mention ionisation in my answer to that one. So I lost a stupid mark there O.o
5bii) I think this is ok too. I think they wanted to see something about detecting smaller changes, or measuring to smaller decimal places, but your answer seems correct to me.
Overall, you don't seem to have dropped that many marks in those questions - only a few! I wouldn't worry about it too much
you'll have done fine