[dopeantelope]

Hi, this is a question on one of my lab reports and was just wondering if i get could get some insight into how i would work this out please.

Background: Synthesized a solid acid, diluted 3.00g of solid acid up to 100ml with distilled water.. titrated with 0.1M NaOH in 30ml aliquots.

Titre average = 11.2ml

Formula in title = H₄SiW₁₂O₄₀

Question:

Assuming the formula given in the title, calculate the moles of titratable acidic protons per mole of compound. Compare this to your experimental results, show the maths for both and comment on the differences.

My attempt:


So to calculate the moles of titratable acidic protons per mole of compound i did,

grams used in titration/Mr of solid acid
3.00g/2976.297 = 0.001mol

Then multiplied that by 4? as there are 4 hydrogens in the molecular formula.
0.001 x 4 = 0.004mol of titratable acidic protons.

Then for my experimental results i have assumed that there is a 1:1 ratio between NaOH and the solid acid. So i worked out the moles in 100ml by using the titration value.

concentration x titre volume/1000 =
0.1 x (11.2/1000) = 0.00112mol

mol divided by aliquot multiplied by total volume
(0.00112/(30/1000)) x (100/1000) = 0.00373mol

then would i not need to multiply that number by 4? That value is close to the 0.004 i calculated in the first step but i cant help feeling im way off the mark.

Can anyone shed some light on this please?

Thanks in advance.

[AWK]

Molecular mass is 2878.1576
Method is OK

[mjc123]

grams used in titration/Mr of solid acid
3.00g/2976.297 = 0.001mol

Then multiplied that by 4? as there are 4 hydrogens in the molecular formula.
0.001 x 4 = 0.004mol of titratable acidic protons.

You were not asked for the moles of titratable acidic protons, but the moles of titratable acidic protons per mole of compound. You have assumed this is 4 - can you give any rationale for this?

then would i not need to multiply that number by 4?

Why? What have you measured?

[dopeantelope]

i assumed it was 4 because of the amount of hydrogens in the molecular formula. is this incorrect?

i guess i wouldnt need to multiply it by 4 as that is the amount of acid that neutralizes the NaOH.

also, i forgot to include waters of crystallization in the formula. The Mr is correct.

[mjc123]

i assumed it was 4 because of the amount of hydrogens in the molecular formula. is this incorrect?

It looks right in this case, but it isn't always - consider e.g. H₃PO₃. And if you had included the water of crystallisation, and the formula was, say, H₈SiW₁₂O₄₂, the answer wouldn't be 8, would it?

i guess i wouldnt need to multiply it by 4 as that is the amount of acid that neutralizes the NaOH.

Correct.

[dopeantelope]

The formula given is H₄SiW₁₂O₄₀.7H₂O, would these hydrogens be included then? So how do i go about finding the moles of titratable acidic protons per mole of compound if my previous calculation is incorrect?

Thanks for your help

[mjc123]

The short answer is that there isn't a short answer - you have to know the chemistry of your compound. It is not obvious, for example, that H₃PO₃ is HP(=O)(OH)₂ and not P(OH)₃. In your case 4 is a good starting point (don't include the water of crystallisation), but the only way to be really sure is to find out.

[AWK]

H₄SiW₁₂O₄₀
Your assumption is correct. All 4 protons are acidic, moreover, this is a strong acid.