Question

For ${H_2}{O_2}$ solution used for hair bleaching is sold as a solution of approximately $5.0\,g\,{H_2}{O_2}$ per $100\,\,ml$ of the solution. The molecular mass of ${H_2}{O_2}$ is $34$. The molarity of this solution is approximately.
A.$0.15\,M$
B.$1.5\,M$
C.$3.0\,M$
D.$3.4\,M$

Answer 1

Here, the molecular mass is given for ${H_2}{O_2}$ is $34$. To calculate the molarity of ${H_2}{O_2}$ solution, first calculate the number of moles of ${H_2}{O_2}$ in $100\,\,ml$ of the solution and then put all values in the formula of molarity.
Formula used:
$Molarity\, = \,\,\dfrac{{Number\,of\,moles\,of\,solute\,(mol)}}{{Volume\,of\,solution\,\,(liters)}}$

Complete answer:
We know that, molarity (M) is defined as the ratio of the number of moles of solute to the volume of solution in liters. Its unit is $moles/liter$. It is used to measure the concentration of a solution.
In other words, molarity indicates the number of moles of solute per liter of solution. It is also used to calculate the volume of solvent or the amount of solute.
$Molarity\, = \,\,\dfrac{{Number\,of\,moles\,of\,solute\,(mol)}}{{Volume\,of\,solution\,\,(liters)}}$
If the solutions has same amount of moles of solute then, it can be represented as ${C_1}{V_1}\, = \,{C_2}{V_2}$ where $C$ is the concentration and $V$ is volume.
According to the question,
Given weight of ${H_2}{O_2}$=$5.0\,g$
Molecular mass of ${H_2}{O_2}$=$34\,g\,mo{l^{ - 1}}$
We know that, $Number\,of\,moles = \dfrac{{Given\,\,weight\,(g)}}{{Molecular\,mass(gmo{l^{ - 1}})}}$
Number of moles of ${H_2}{O_2}$=$\dfrac{5}{{34}}\,\,mol$
Number of moles of ${H_2}{O_2}$= $0.147\,mol$
It is given that,
Volume of ${H_2}{O_2}$ solution =$100\,\,ml$= $0.1\,L$
Using the formula for molarity,
$Molarity\, = \,\,\dfrac{{Number\,of\,moles\,of\,solute\,(mol)}}{{Volume\,of\,solution\,\,(liters)}}$
Molarity = $\dfrac{{0.147}}{{0.1}}$
Molarity =1.47\,mol\,{L^{ - 1}}$$$$ \approx 1.5\,mol\,{L^{ - 1}}
Hence, the molarity of solution is $1.5\,mol\,{L^{ - 1}}$
The correct answer is option (B).

Note:
Don’t get confused between molarity and molality. There is a difference between molarity and molality. Molality is defined as the number of moles of solute dissolved in one Kilogram of solvent. It is denoted by m. Its unit is $mol/Kg$. It is also used to determine the concentration of solutions. It gives very precise and accurate values. However, molarity may be imprecise and accurate.