Question

How do you calculate the number of moles from volume?

Answer 1

Calculate the number of moles from the volume:

• For calculating the number of moles from the volume, we first have to decide which type of substance is given to us.
• If we are given a pure liquid or a solid, we have to use its density to calculate its mass and then divide the mass by the molar mass.
• If we are given a solution, we have to multiply the molarity by the volume in liters.

Moles from the density of pure solid or liquid:

• For calculating the moles from the volume of pure liquid or solid first, we have to Multiply the volume by the density to get the mass.
• Then Divide the mass by the molar mass to get the number of moles.
• Example: Calculating the moles in 10.00 mL of acetic anhydride; (Given: The molar mass of acetic anhydride is 102.1 g/mol and its density is 1.080 g/mL)
• Here, The formula for density is:

$$\begin{gathered} \Rightarrow \mathrm{\rho }=\mathrm{vm} \\ \Rightarrow \mathrm{m}=\mathrm{V}\times \mathrm{\rho } \\ \Rightarrow \mathrm{mass}=10.00\mathrm{mL}\times 1.080\mathrm{g}{(1\mathrm{mL})}^{-1}=10.80\mathrm{gm} \end{gathered}$$ (​where ρ is the density, m is the mass and V is the volume of the sample)

• Hence, the number of moles is given by; $\mathrm{n}=10.8/102.1=0.1058\mathrm{mol}.$

Moles from the volume of solution:

• Molarity is stated as the number of moles of a substance in one liter of solution given by;

$\mathrm{M}=\mathrm{Vn}$ ​(where n is the number of moles and V is the volume in liters)

• We can rearrange this equation to get the number of moles:$\mathrm{n}=\mathrm{M}\times \mathrm{V}$
• Example: Calculating no moles of NaCl contained in 0.300L of 0.400 mol/L NaCl Solution
• Here, no moles of NaCl by using the formula is;

$$\begin{gathered} =0.300\mathrm{L}.\mathrm{soln}\times \frac{1\mathrm{L}\mathrm{soln}}{0.400\mathrm{mol}\mathrm{NaCl}} \\ =0.120\mathrm{mol}\mathrm{NaCl} \end{gathered}$$

Hence, By using the above two mentioned ways we can calculate the number of moles from volume.