Copper(II) is determined iodometrically: $2 Cu X^{2 +} + 4 I X^{-} 2 CuI + I X_{2}$ , then $I X_{2}$ is titrated with thiosulphate. A sample liberates iodine that needs 20.0 mL of 0.100 M $Na X_{2} S X_{2} O X_{3}$ . The mass of copper in the sample is (Cu = 63.5):

Answer: C

mmol thiosulphate =

20.0 \times 0.100 = 2.0

I X_{2}

= 1.0 mmol (2 thiosulphate per

I X_{2}

), and

Cu X^{2 +}

= 2.0 mmol (2

Cu X^{2 +}

per

I X_{2}

). Mass =

2.0 \times 63.5 = 127

mg = 0.127 g.