Question

What is the molarity of pure water at $4°\mathrm{C}$?

Answer 1

1. Molarity is defined as the number of moles of solute present per liter of the solution.
2. There are various ways of calculating the concentration or the strength of the solution and molarity is one of them.
3. Mathematically molarity can be represented as follows: $\mathrm{Molarity}\left(\mathrm{M}\right)=\frac{\mathrm{No}.\mathrm{of}\mathrm{moles}\mathrm{of}\mathrm{solute}\left(\mathrm{n}\right)}{\mathrm{Volume}\mathrm{of}\mathrm{the}\mathrm{solution}\left(\mathrm{V}\right)}$

Step 1: Given data

• Density of pure water is $\frac{1\mathrm{Kg}}{1\mathrm{ml}}$.
• But the molarity is calculated in terms of litre then we have to convert the milliliter to Litre.
• $\frac{1\times 1000\mathrm{g}}{1\times 1000\mathrm{ml}}=\frac{1\mathrm{Kg}}{1\mathrm{L}}$

Step 2: Calculating the number of moles

• Molecular mass of water $\left(H_{2}O\right)$ is $16+2=18$
• Number of moles can be calculated as:$\mathrm{n}=\frac{\mathrm{Given}\mathrm{mass}}{\mathrm{Molecular}\mathrm{mass}}=\frac{1\mathrm{Kg}}{18}$
• $\mathrm{n}=\frac{1000}{18}=55.56\mathrm{mol}$
• We get, $\mathrm{n}=55.56\mathrm{moles}$

Step 3: Calculating the molarity

• The volume of the solution is$1\mathrm{L}$, therefore the molarity is;
• $\mathrm{Molarity}\left(\mathrm{M}\right)=\frac{\mathrm{No}.\mathrm{of}\mathrm{moles}\mathrm{of}\mathrm{solute}\left(\mathrm{n}\right)}{\mathrm{Volume}\mathrm{of}\mathrm{the}\mathrm{solution}\left(\mathrm{V}\right)}=\frac{55.56}{1\mathrm{L}}$
• Therefore the molarity of water at $4°\mathrm{C}$ is $55.56{\mathrm{molL}}^{-1}$.