What are three possible errors that could be introduced

Question

BaCl₂ + Na₂SO₄ ↔BaSO₄ + NaCl- Gravimetic determination of sulfate1) Explain, why adding an excess of Ba⁺² will causea more complete precipitation of BaSO₄.2) What are three possible errors that could be introduced into this procedure by poor experimneter technique?3) If some of the solid precipitated were actually SrSO₄ instead of BaSO₄, would the final %sulfate calculated be higher or lower than actual value?Explain.