[islamike]

Hi everyone,

I don't really know if I am doing this question right:

2.954g of a dicarboxylic acid Q was dissolved into 500cm³ of solution. A portion of 25.0cm³ of the solution was then taken and required 27.3cm³ of sodium hydroxide solution at 0.104moldm³. Determine the formula of the dicarboxylic acid Q.

I used this balanced equation: C₂H₂O₄ + 2NaOH  NaC₂O₄ + 2H₃O

and calculated the Mr of C₂H₂O₄ to be 104.043. Dividing this answer by the Mr(C₂H₂O₄) gives me 1.156....
Now I am not sure if this is correct.

Could someone please verify that the balanced equation is correct and that the Mr I obtained is correct for this question.
Any help is appreciated

Thanks in advance.

[thetada]

Hi there, welcome to the forum. Before you think about the relative mass of the acid, you need to start by working out the number of moles. To work out the equation, which you very sensibly considered, you can let the formula of the acid be H₂X, where X is the dicarboxylate.

[Borek]

You are doing strange things (and using C₂H₂O₄ as a formula doesn't help, as thedata wrote, using symbolic formula of H₂X would be much better, or - taking into account fact the infomation given - X(COOH)₂).

However, 104.0 g/mol is a correct molar mass of the acid. Now, assuming the formula to be X(COOH)₂), how many grams per "mole of X"?

[thetada]

Sorry, you were further ahead in the calculation than I realised. Thanks Borek

[islamike]

You are doing strange things (and using C₂H₂O₄ as a formula doesn't help, as thedata wrote, using symbolic formula of H₂X would be much better, or - taking into account fact the infomation given - X(COOH)₂).

However, 104.0 g/mol is a correct molar mass of the acid. Now, assuming the formula to be X(COOH)₂), how many grams per "mole of X"?

So, the gmol^-1 of X(COOH)₂ 104? In this case I need to take 94 gmol^-1 of (COOH)₂ away from 104, which is 10.  But what does X represents here? an element, Beryllium or Boron? or perhaps 10H (Hydrogen)?

[thetada]

I'm not sure if your calculation of 94g mol^-1 is correct. The leftover should be composed of carbon and hydrogen atoms.

[islamike]

I'm not sure if your calculation of 94g mol^-1 is correct. The leftover should be composed of carbon and hydrogen atoms.

Sorry my bad, 94gmol^-1 should be 90gmol^-1.

So, the final formula is H₂C(COOH)₂ ?
I think that makes sense.

Thank you both. Much appreciated

[islamike]

Wondering what will be the balanced equation?

Since my furmula, C2H2O4 , was wrong and working out that the dicarboxylic acid now has the formula H₂C(COOH)₂, the balanced equation: C₂H₂O₄ + 2NaOH  NaC₂O₄ + 2H₃O is wrong.

Looking at my work, I am confused how I obtained 104.043, since my equation wasn't balanced. What balanced equation should I have used

and what is the balanced equation after finding that dicarboxylic acid has formula C₃H₄O₄ ?

[thetada]

Your equation was wrong but it still had the correct Stoichiometry, namely 1 Mole of acid to 2 moles of alkali. Questions like this are common ( at least in UK A- level) the trick is to let acid be H₂X and base the Stoichiometry on the number of protons available from the acid and the number of hydroxide ions on the base, which is effectively what you did

[islamike]

Thanks Thedata,

your help is appreciate 

[thetada]

My pleasure :-)