GCSE Chemistry help!Watch
As per part a, I would say your method seems correct:
50 cm^3 / 1000 = 0.05 dm^3
0.05 dm^3 * (0.025 mol / dm^3) = 0.00125 moles
And for part b: it is something I have not specifically attempted or learnt before. However, I would like to use some application of the Avogadro constant:
" The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 10^23 per mole. "
Using that you may assume that 0.00125 * (6.02 * 10^23) = number of moles of ions from magnesium nitrate.
Although all of the above is just some speculation- like you, I am just a GCSE student at the moment;
Hope it helps!