# Stoichiometry question from Test di Medicina

Tooth enamel consists mostly of hydroxyapatite, whose formula unit is Ca5(PO4)3(OH). In an acidic medium of sulphuric acid, it can be decomposed according to the following reaction:

Ca5(PO4)3(OH) + 4H+ → 5(Ca)2+ (+) 3(HPO4)2- + H2O

How many moles of H2SO4 are needed to produce 0.3 moles of ions?

A. 0.2
B. 0.4
C. 0.3
Q. 0.1
E. 0.5

This seems a bit different then usual stoichiometry questions and I couldn’t figure out how to approach and solve it. Maybe someone can help?

could you tell me which year’s question this one is. i want to read the Italian version as there are usually translation errors as well

My apologies, I forgot to mention it. This is year 2019, question 47, and I used the test from testbusters.it

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no problems!
here is how you can solve it:
in order to have 0.3 moles of HPO-2 the amount of H+ you need is
0.3* 4/3= 0.4 mol H+
as we know that H2SO4 is a diprotic molecule, it will donate 2H+ molecules instead of one.
that is why we divide 0.4 mol/2 to get the amount we want.

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When it says produce ‘0.3 moles of ions’ its just referring to the HPO4 ions then?
Does that mean the Ca2+ ions are not relevant because the H2SO4 is not linked to the number of Ca2+ produced?

yes! i believe the problem is the translation. i tried to find the solution via Italian version