Question

We have $5g$ of $HBr$ in $100ml.$ Find $pH$ .

Answer 1

We must recall the basic definition of molarity and we must also keep in mind the correlation of volume and density of a solution. Molarity is the number of moles of solute per litre of solution.

Complete answer:
Molar concentration and $pH$ can be used to convert between the mass or moles of solute and the volume of the solution. Molarity is a measurement of the moles in the total volume of the solution, whereas Molality is a measurement of the moles in relationship to the mass of the solvent. Hence, the two should never be confused.
Let us first assume that there is $100ml$ of the resulting solution. So, we can write the given amount as $5g$ , We know, The solution contains $5g$ of $HBr$ which means the mass of $HBr$ in the solution is:
$5g~\left( HBr \right)=?mole~$ , molecular weight $H=1,\text{ }Br=79.904$ ,
total molecular mass $=80.904$ ; total molecular mass $=80.904$ ,
we know that, $mass=mole\times molar mass$
here, $\Rightarrow mole=\dfrac{5g}{80.904}=~0.062$
there are $0.062$ moles in $100\text{ }ml$ along with $0.1\text{ }L=0.62\text{ }M$ .
In $HBr$ , there are $0.62\text{ }M$ of ${{H}^{+}}~$ and $0.62\text{ }M$ of $Br$
Thus, $pH$ is given by; $pH=-log\left[ {{H}^{+}} \right]=-log\left[ 0.62 \right]=0.2$

Note:
We should use Molality instead of Molarity in experiments involving significant temperature changes. This is because as the volume of a solution increases with temperature, heating causes molarity to decrease; however, since molality is based on mass rather than volume, molality remains unchanged.