Question

A mixture of $HCl$ and ${H_3}P{O_4}$ is titrated with $1.0M\,NaOH$ .The first end point (Methyl red) occurs at $35.0mL$ and second endpoints (Bromothymolblue) occurs at $50.0\,mL$ .($15.0mL$ after the first end point). The number of millimoles of $HCl$present in the solution is___?

Answer 1

Acid‐base titration is one of the most common operations in analytical chemistry. A solution containing an unknown amount of ionizable hydrogen can be titrated with a solution of standard base until all the hydrogen ions have been consumed .

Complete answer: Titration of $HCl\,\& \,{H_3}P{O_4}$ Mixture Using Indicators and a pH Meter.
The amount of $NaOH$ used by phosphoric acid. = $15mL$
Therefore amount of phosphoric acid present is $15mL \times 1M$=$15\,{\text{millimoles}}$
[$\because molarity = 1M$]
During the first end point, out of $35mL$ , $15mL$ are used for neutralization of phosphoric acid.
$\therefore$ The amount of $NaOH$ used for neutralization of $HCL$ ,
${\text{HCl = 20 mL}} \\\ {\Rightarrow {HCl = 20mL \times 1M}} \\\ {\Rightarrow \text{HCl = 20 millimoles}} \\\$

Hence the number of millimoles of $HCl$ present in the solution is 20millimoles.

Additional Information: Double titration:
The method involves two indicator phenolphthalein and methyl orange. This is a titration of specific compounds. Let us consider a solid mixture of $NaOH,{\text{ }}N{a_2}C{O_3}$ and inert impurities weighing. You are asked to find out the % composition of the mixture. You are also given a reagent that can react with the sample, say, $HCL$ along with its concentration (${M_1}$).
We first dissolve the mixture in water to make a solution and then we add two indicators in it, namely phenolphthalein and methyl orange. Now, we titrate this solution with $HCL$ .
$NaOH$ is a strong base while $N{a_2}C{O_3}$ is a weak base. So it is safe to assume that $NaOH$ reacts with $HCL$ first, completely and only then does $N{a_2}C{O_3}$ react.

Note: A titration is a technique where a solution of known concentration is used to determine the concentration of an unknown solution.Typically,the titrant(the known solution) is added from a burette to a known quantity of analyte (the unknown solution) until the reaction is complete.