# Imat 2021 question 51

the answer is A , I just want to make sure about the reason why number 1 is incorrect
is it because it says ‘‘the ion concentration’’ ?how can we alter the sentence to make it correct?

Hey Yasy,

the hydrogen ion concentration of Y = 0.1 mol/dm3 (because there are two H+ ions in the salt H2SO4). The hydrogen ion concentration of X = 0.05 mol/dm3, because there is only one H+ ion in HNO3. Therefore the hydrogen ion concentration in Y is only 2 times higher than in X.

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I didn’t actually understand your calculation but if the 4 is incorrect then what is wrong with this calculation precisely?
there is 50 ml of 0.05mole/L , there will be 50*0.05/1000 = 2.5 10^-3 mole HNO3, due to that they both are strong acids that number is the exact number of H here ( 1 mole)
Y on the other hand 100ml 0.05mole /1000ml =510^-3
we have 2 moles so 5
10^-3 *2 = 10^-2
10^-2 / 2.5 *10^-3 =100/25=4

I think you calculated the moles of hydrogen ions in both solutions, but they are asking about the concentration. So, we need to divide those moles by the volume of the solutions.

The quick trick I use when solving these problems is this:

H2SO4 has a given concentration, but we know it dissociates to give 2H+ and 1SO4-. Thus the concentration of H+ (for same volume of solution), is twice the concentration of H2SO4.

This way, you can find the concentration of ions quickly and compare them between different solutions:) I hope it makes sense, but feel free to ask if you do not understand something!

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thank you, that made it crystal clear!

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