We’re being asked to calculate the **molarity (M)** of a solution of NaI. Recall that ** molarity** is the ratio of the moles of solute and the volume of solution (in liters). In other words:

$\overline{){\mathbf{Molarity}}{\mathbf{}}{\mathbf{\left(}}{\mathbf{M}}{\mathbf{\right)}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}\frac{\mathbf{moles}\mathbf{}\mathbf{of}\mathbf{}\mathbf{solute}}{\mathbf{Liters}\mathbf{}\mathbf{of}\mathbf{}\mathbf{solution}}}$

We first need to determine the number of moles of NaI since this is the only information missing for us to find molarity. Notice that the volume of the solution in liters is already given.

We’re given the mass of NaI, **10.7**** g**. We can use the molar mass of NaI to find the moles. The molar mass of NaI is

The** moles of NaI** is:

$\mathbf{10}\mathbf{.}\mathbf{7}\mathbf{}\overline{)\mathbf{g}\mathbf{}\mathbf{NaI}}\mathbf{}\mathbf{\left(}\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{NaI}}{\mathbf{149}\mathbf{.}\mathbf{89}\mathbf{}\overline{)\mathbf{g}\mathbf{}\mathbf{NaI}}}\mathbf{\right)}\mathbf{=}$**0.0714 mol**

We know that the volume of the solution is **0.250 L**. We can now calculate for **molarity**:

What is the molarity of a solution prepared by dissolving 10.7 g Nal in 0.250 L?

A) 42.8

B) 0.0714

C) 2.86 x 10^{-4 }

D) 0.286

E) none of the above

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