Respuesta :
When calculating concentration, we always need to remember that the number of moles remain constant.
The formula for concentration is:
[tex]c=\frac{n}{V}[/tex]So, if we don't add or remove solute, n will remain constant, while c and V vary.
If we want a solution of chloride ion concentration of 0.100 mol/L and 250 mL of volume, we need to first convert 250 mL to L:
[tex]250mL=\frac{250}{1000}L=0.250L[/tex]Now, we can use the formula for concentration to calculate the number of moles:
[tex]\begin{gathered} c_{Cl^{-}}=\frac{n_{Cl^{-}}}{V_{Cl^{-}}} \\ n_{Cl^{-}}=c_{Cl^{-}}V_{Cl^{-}} \\ n_{Cl^{-}}=0.100mol/L\cdot0.250L=0.0250mol \end{gathered}[/tex]Now, the dissociation of MgCl₂ will be:
[tex]MgCl_2\to Mg^{2+}+2Cl^-[/tex]So, for each mol of MgCl₂, we will have 2 moles of Cl⁻.
So, if we need 0.0250 mol of Cl⁻, we need half as many of MgCl₂, that is:
[tex]n_{MgCl_{2}}=\frac{1}{2}n_{Cl^-}=\frac{1}{2}\cdot0.0250mol=0.0125mol[/tex]Now, we need to calculate the volume necessary of the solution of 0.500 mol/L MgCl₂:
[tex]\begin{gathered} c_{MgCl_{2}}=\frac{n_{MgCl_2}}{V_{MgCl_{2}}} \\ V_{MgCl_2}=\frac{n_{MgCl_2}}{c_{MgCl_{2}}}=\frac{0.0125mol}{0.500mol/L}=0.0250L=0.0250\cdot1000mL=25mL \end{gathered}[/tex]So, we will need 25 mL os the solution.
