Question

a 5.0 g sample of mgcl₂ may contain measurable amounts of other compounds as impurities. which of the following quantities is (are) needed to determine that the sample is pure mgcl₂? a the color and density of the sample b the mass of mg in the sample only c the number of moles of cl in the sample only d the mass of mg and the mass of cl in the sample

Explanation:

To determine if a \( \ce{MgCl_2} \) sample is pure, we need to check the ratio of Mg to Cl, which is fixed in pure \( \ce{MgCl_2} \) (1:2 in moles, or based on molar masses).

• Option A: Color and density are physical properties; impurities might not change them enough to confirm purity.
• Option B: Only mass of Mg can’t confirm, as impurities could have Mg (e.g., \( \ce{MgO} \)) or affect Cl ratio.
• Option C: Only moles of Cl can’t confirm, as impurities could have Cl (e.g., \( \ce{NaCl} \)) or affect Mg ratio.
• Option D: With mass of Mg and Cl, we can calculate their moles, check the mole ratio (should be 1:2 for \( \ce{MgCl_2} \)), and verify against the sample’s total mass (5.0 g) to ensure no other elements/compounds are present.

Answer:

D. The mass of Mg and the mass of Cl in the sample