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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?

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  • $\begingroup$ Look up the difference between molar and molal solutions. $\endgroup$ – user55119 Aug 8 at 19:58
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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).

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