Sodium fluoride and lead nitrate are both soluble in water. If 1.13 g of NaF and 0.232 g of Pb(NO3)2 are dissolved in sufficient water to make 1.00 L of solution, will a precipitate form? How do you know if there should be a precipitate or not? What is the chemical formula for the possible precipitate? Look in your textbook for any necessary equilibrium constants.
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To determine whether a precipitate will form when sodium fluoride (NaF) and lead nitrate (Pb(NO3)2) are dissolved in water, we need to examine the solubility of the resulting compounds.
1. Calculate the molar masses of NaF and Pb(NO3)2:
- Molar mass of NaF = molar mass of Na (22.99 g/mol) + molar mass of F (18.998 g/mol) = 40.99 g/mol
- Molar mass of Pb(NO3)2 = molar mass of Pb (207.2 g/mol) + 2 × (molar mass of N (14.007 g/mol) + 3 × (molar mass of O (15.999 g/mol))) = 331.2 g/mol
2. Convert the given masses of NaF and Pb(NO3)2 to moles:
- Moles of NaF = mass of NaF / molar mass of NaF = 1.13 g / 40.99 g/mol ≈ 0.0276 mol
- Moles of Pb(NO3)2 = mass of Pb(NO3)2 / molar mass of Pb(NO3)2 = 0.232 g / 331.2 g/mol ≈ 0.0007 mol
3. Calculate the concentration of each compound in the final solution:
- Sodium fluoride (NaF): [NaF] = moles of NaF / volume of solution = 0.0276 mol / 1.00 L = 0.0276 M (from the given information)
- Lead nitrate (Pb(NO3)2): [Pb(NO3)2] = moles of Pb(NO3)2 / volume of solution = 0.0007 mol / 1.00 L = 0.0007 M (from the given information)
4. Determine the solubility of each compound using solubility rules or consulting a solubility table or your textbook.
According to the solubility rules:
- Most nitrate (NO3-) salts are soluble.
- Most fluoride (F-) salts are soluble, except those containing Group 2 cations (including Pb2+).
Based on these rules, Pb(NO3)2 is soluble, but NaF is not soluble in water due to the presence of the lead ion (Pb2+). Therefore, a precipitate should form.
5. Determine the chemical formula for the precipitate:
When NaF and Pb(NO3)2 react, a double replacement reaction occurs, resulting in the following reaction:
NaF + Pb(NO3)2 → NaNO3 + PbF2
The chemical formula for the precipitate is PbF2.
6. Check your textbook or other reliable sources for equilibrium constants if needed. In this case, the solubility product constant (Ksp) of PbF2 is necessary to calculate the equilibrium concentration of the precipitate.