# One mole of solute (NaCl) is dissolved in 1 litre water. The molarity of solution is [closed]

One mole of solute ($$\ce{NaCl}$$) is dissolved in $$\pu{1 L}$$ of water. The molarity of the solution is:

A) $$\pu{<1M}$$

B) $$\pu{>1M}$$

C) $$\pu{=1M}$$

D) $$\pu{=2M}$$

My answer: It’s a fairly simple question so, I did what anybody would do.

$$\mathrm{Molarity}=\frac{\text{No. of moles}}{\text{volume of solution}}$$

$$\mathrm{Molarity}=1$$

That is the obvious answer, and it’s probably right, but the answer given is $$\pu{<1M}$$. I just wanted to confirm that the answer given is right or wrong. If it is right, why?

• Look up the difference between molar and molal solutions. – user55119 Aug 8 at 19:58

## 1 Answer

The answer $$<\pu{1M}$$ is correct.

Suppose you have $$\pu{1L}$$ ($$\pu{1dm^3}$$) of water and then you add your salt. When you add your salt the volume of the solution increases so now you have $$\pu{1 mol}$$ of $$\ce{NaCl}$$ in more than $$\pu{1L}$$ of solution. That means that you can't have a $$\pu{1M}$$ solution anymore.

If you want a $$\pu{1M}$$ solution then:

1. Add $$\pu{1 mol}$$ of salt in a flask.
2. Add water until you reach $$\pu{1L}$$ of volume.

EDIT: when you add salt, the volume of the solution increases slightly and less than what you expect(volume of water + volume of dry salt).