1. Mass/Mr = Mole
So 25/40.1+12+[16x3] = 0.25 mole of CaCO3 you times this by 2 because the molar ratio of CaCO3 to HCL is 1:2. so 0.5 mol of HCL.
Volume = Mole/Concentration , you worked out the mole and the concentration is given to you in the question [0.100 mol dm-3] > 0.5/0.100 = 5 dm3
2. first of all because the questions says that its monoprotic means it only has 1 H, therefore it can only be HCL or HNO3. so the equation will be : HX + NaOH -> NaX + H2O
the molar ratio is 1 : 1 : 1.
first you work out the mole of NaOH, which is concentration x volume = mole
0.250 x [23.5/1000] = 5.875x10-3 mole of NaOH , and because its 1:1 the mole is the same for HX
and to work out the Mr =Mass/Mole, 13.8/[5.875x10-3] = 2348.936170...etc which is 2349 gmol-1
i'm doing the same questions for homework and that's what i've done, hope it helps