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.

🤓 Based on our data, we think this question is relevant for Professor Poehlmann & Waymouth's class at STANFORD.

FREE Expert Solution

Recall molarity (M):

molarity(M)=moles soluteL solution


Calculate the total moles of F- used:

moles F-=1.00 L×1.53 mol NaFL×1 mol F-1 mol NaF

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

mass CrF3+mass MgF2=50.0 gx(109.0gmol)+y(62.3gmol)=50.0 g109.0x+62.3y=50.0 gisolate y:62.3y62.3=50.0 g-109.0x62.3y=50.0 g-109.0x62.3


Substitute in equation 1:

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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.

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Based on our data, we think this problem is relevant for Professor Poehlmann & Waymouth's class at STANFORD.