neutralization/titration - calculation error?

[MDwannabe7]

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Sodium carbonate is the preferred primary standard for acidic solutions. What is the mass of sodium carbonate required to neutralize 25.10 mL of 0.07520 N HCl(aq)? (The molecular weight of sodium carbonate is 106.0 amu.)


In order to neutralize HCl, the moles of carbonate would need to equal the moles of H+. Therefore, if you convert the 25.10 mL to L and multiply by the normality (basically equal to molarity since for HCl, the stuff of interest is only 1 H+). This gives you the number of moles of HCl. If you then do a stoichiometric ratio of HCl to Na2CO3 (which I think is 1 to 1, since there is only 1 CO3 per Na2CO3) and then multiply by the molecular weight of sodium carbonate, you should the the necessary grams that the question is asking for.

When I did this, I obtained 200.0 mg, but the answer is actually 100.0 mg. I assumed that my mistake was that in the stoichiometric ratio of how many moles of sodium carbonate it takes to neutralize 1 mole of HCl, that it should actually be 2 to 1 instead of 1 to 1. However, if I plug that in, I get 400.0 mg, not 100.0 mg.

Is Kaplan just incorrect, or did I set this up wrong - any ideas?

 

[not respect]

carbonate ion can take two equiv of protons to carbonic acid.

 

[MDwannabe7]

carbonate ion can take two equiv of protons to carbonic acid.

Okay, but if I set up the ratio of 2 carbonates per HCl - I get 400 mg, not 100 - what am I doing wrong?

 

[not respect]

okay when you use only 1 equivalent you get 200 but if you use two then how would that be 400? it should be less not more.

 

[MDwannabe7]

okay when you use only 1 equivalent you get 200 but if you use two then how would that be 400? it should be less not more.

Please look at the picture I attached with my most recent post - it shows the stoichiometric ratio that I set up. Conceptually, I understand what you are saying, but come MCAT time, I tend to second guess myself and will rely on such ratios in which the units cancel - so where did I go wrong in the ratio?

 

[not respect]

okay well if one carbonate anion can take two equivalents of protons, then your ratio should be 2 HCl : 1 Na2CO3, not the other way around like you have it.

 

[MDwannabe7]

okay well if one carbonate anion can take two equivalents of protons, then your ratio should be 2 HCl : 1 Na2CO3, not the other way around like you have it.

Don't know why I'm so dense sometimes - thanks guys!