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Solution calculations

How do I select the correct values in solution calculations e.g. 3.88g of a monoprotic acid was dissolved in water and the solution made up to 250cm3. 25.0cm3 of this solution was titrated with 0.095 mol dm-3 NaOH solution, requiring 46.5cm3. Calculate the relative molecular mass of the acid?
(edited 8 years ago)
First of all, a balanced equation. A monoprotic acid is simply one hydrogen atom per molecule of acid, such as HCl or HNO3. You probably knew that, but I'm just putting it out there in case, so the next bit makes sense. Let's call the non-hydrogen part of the molecule X, so the above acids are represented by HX. An ion of X must have a charge of -1.

HX(aq) + NaOH(aq) ---> Nax(aq) + H2O(l)
3.88g 0.095 mol dm-3

Then, write all the info you have around the equation as I did above.

The only thing you can work out at the moment is the moles of the NaOH (the alkali).
moles= volume x concentration so moles of NaOH = 0.0465 x 0.095 = 0.0044175 moles (I used 0.0465 because you need to convert cm cubed into dm cubed).

We see from the equation that 1 mole of HX reacts with 1 mole of NaOH, so 0.0044175 moles of HX react with 0.0044175 of NaOH.

Now, you can work out the Mr (molecular mass) of HX using Mr=mass/moles. 3.88g/0.0044175 = 878.3248444

In chemistry we round to 3 sig. fig., so round it to 878. That's really quite heavy, but you can probably work out what X is from that.

Hope I'm not answering this too late. Bottom line is, write out the equation and all the info you have first.

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