There are some problems I don't understand how they are solved (I have the answer). Since I can't find anything related to these on the internet, I'm asking about the problem and the answer here.
Problem:
For the titration of $\mathrm{25~mL}$ solution which has $\ce{H3PO4}$ and $\ce{H2SO4}$ in it, there are spent $\mathrm{37.2~mL~0.1008~M~NaOH}$ in the presence of methyl orange as an indicator. The other $\mathrm{25~mL}$ of the solution spent $\mathrm{49.6~mL~NaOH}$ in the presence of phenolphthalein as an indicator. Find the concentration of both acids.
Answer:
In the presence of methyl orange, will happen these reactions:
- $\ce{H2SO4 + 2 NaOH -> Na2SO4 + 2H2O}$
- $\ce{H3PO4 + NaOH -> NaH2PO4 + H2O}$
And there are spent $\mathrm{37.2~mL~NaOH}$.
In the presence of phenolphthalein, the following reactions will happen:
- $\ce{H2SO4 + 2NaOH -> Na2SO4 + 2H2O}$
- $\ce{H3PO4 + 2NaOH -> Na2HPO4 + 2H2O}$
And there are spent $\mathrm{49.6~mL~NaOH}$.
Question:
The procedure to find the concentration of the acids is obviously easy, but what I don't understand is the volumes of $\ce{NaOH}$ we should take to make the calculations.
The volume of $\ce{NaOH}$ needed for $\ce{H3PO4}$ is $\mathrm{49.6-37.2=12.4~mL}$.
The volume of $\ce{NaOH}$ in presence of methyl orange, needed for $\ce{H2SO4}$ is $\mathrm{37.2-12.4=24.8~mL}$, and in the presence of phenolphthalein is $\mathrm{49.6-24.8=24.8~mL}$.
There are also more similar problems with different compounds, but I totally don't understand the logic how these volumes are calculated! How do you know which one to deduct?
If you also know any website with these problems please send me the link, thanks.