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+)
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:
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.
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