First, determine the mass of sulfate in the sample. To do this, you need the following information: – Molar mass of SO4^2-: 96.06 g/mol – Molar mass of BaSO4 (which is your solid precipitate): 233.43 g/mol Mass of SO4^2- in sample (in g) = (g BaSO4) * (mol BaSO4 / 233.43 g BaSO4) * (mol SO4^2- / mol BaSO4) This is the mass of your precipitate after oven drying. Second, determine the mass percent of sulfate in the sample: Mass percent of SO4^2- in the sample = (mass of SO4^2- in the sample (in g) / mass of original soluble sulfate sample (in g)) * 100 Third, compare your experimental mass percent of sulfate (above) to the mass percentages of sulfate that were calculated in the Procedure in Step #2, page 1. Identify your original soluble sulfate-containing unknown as either Na2SO4, K2SO4, or (NH4)2SO4. Last, evaluate the accuracy of your mass percent sulfate result by calculating a percent error: Percent Error = (|your experimental mass % sulfate – actual mass % sulfate| / actual mass % sulfate) * 100
The Correct Answer and Explanation is:
Correct Answer (Example):
Assume the mass of oven-dried BaSO₄ precipitate = 0.467 g
Assume the original sample mass = 0.300 g
Step 1: Determine mass of SO₄²⁻ in the sampleMass of SO₄²⁻=0.467 g BaSO₄×(1 mol BaSO₄233.43 g BaSO₄)×(96.06 g SO₄²⁻1 mol BaSO₄)\text{Mass of SO₄²⁻} = 0.467\ \text{g BaSO₄} \times \left(\frac{1\ \text{mol BaSO₄}}{233.43\ \text{g BaSO₄}}\right) \times \left(\frac{96.06\ \text{g SO₄²⁻}}{1\ \text{mol BaSO₄}}\right)Mass of SO₄²⁻=0.467 g BaSO₄×(233.43 g BaSO₄1 mol BaSO₄)×(1 mol BaSO₄96.06 g SO₄²⁻)Mass of SO₄²⁻=0.467×96.06233.43=0.1922 g\text{Mass of SO₄²⁻} = 0.467 \times \frac{96.06}{233.43} = 0.1922\ \text{g}Mass of SO₄²⁻=0.467×233.4396.06=0.1922 g
Step 2: Determine mass percent of SO₄²⁻ in the sampleMass % SO₄²⁻=(0.19220.300)×100=64.07%\text{Mass \% SO₄²⁻} = \left(\frac{0.1922}{0.300}\right) \times 100 = 64.07\%Mass % SO₄²⁻=(0.3000.1922)×100=64.07%
Step 3: Compare to known values for identifying unknown
- Na₂SO₄ theoretical % SO₄²⁻: 56.48%
- K₂SO₄ theoretical % SO₄²⁻: 54.02%
- (NH₄)₂SO₄ theoretical % SO₄²⁻: 72.68%
The experimental result of 64.07% is closest to (NH₄)₂SO₄ (ammonium sulfate), but still somewhat off. Based on proximity, the unknown sample is most likely (NH₄)₂SO₄.
Step 4: Calculate percent errorPercent error=∣64.07−72.6872.68∣×100=∣−8.6172.68∣×100=11.85%\text{Percent error} = \left|\frac{64.07 – 72.68}{72.68}\right| \times 100 = \left|\frac{-8.61}{72.68}\right| \times 100 = 11.85\%Percent error=72.6864.07−72.68×100=72.68−8.61×100=11.85%
Explanation:
To identify the unknown sulfate salt, the sulfate ion content must be extracted via a precipitation reaction. Barium sulfate forms as a precipitate when barium chloride is added to a solution containing sulfate ions. This precipitate is collected and dried. By converting grams of BaSO₄ into moles, and then into grams of SO₄²⁻, the actual sulfate content in the original sample is revealed. Calculating the percent mass of sulfate in the sample allows comparison with known sulfates. The identity of the salt can be inferred by matching the experimental sulfate percentage with theoretical values. Percent error further assesses the experiment’s accuracy
