# Problem: A solution contains Cr3 +  ion and Mg2 +  ion. The addition of 1.00 L of 1.53 M NaF solution is required to cause the complete precipitation of these ions as CrF3 (s) and MgF2 (s). The total mass of the precipitate is 50.0 g .Find the mass of Cr3 +  in the original solution.

###### FREE Expert Solution

Recall molarity (M):

Calculate the total moles of F- used:

moles F- = 1.53 mol F- → total moles

Cr3+ + 3 F- → CrF3

Mg2+ + 2 F- → MgF2

total moles = 3(moles Cr3+) + 2(moles Mg2+)

let:

moles Cr3+ = x
moles Mg2+ = y

3x + 2y = 1.53 mol (equation 1)

mass CrF3 + mass MgF2 = 50.0 g

molar mass CrF3 = 109.0 g/mol

molar mass MgF2 = 62.3 g/mol

1 mole Cr3+ : 1 mol CrF3 = x
1 mole Mg2+ : 1 mol MgF2 = y

Substitute in equation 1:

86% (195 ratings) ###### Problem Details

A solution contains Cr3 +  ion and Mg2 +  ion. The addition of 1.00 L of 1.53 M NaF solution is required to cause the complete precipitation of these ions as CrF3 (s) and MgF2 (s). The total mass of the precipitate is 50.0 g .

Find the mass of Cr3 +  in the original solution.