Question

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

Answer 1

Hint: 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.