For a certain negative sol the flocculation values are 50 mmol/L for $NaCl$ and 0.80 mmol/L for $CaCl X_{2}$ . Assume that the coagulating effects of the two salts are additive, i.e. coagulation occurs when $c_{1} / FV_{1} + c_{2} / FV_{2} = 1$ . If the sol already contains 20 mmol/L of $NaCl$ , what additional concentration of $CaCl X_{2}$ (in mmol/L) just coagulates it? (two decimals)

Numerical answer type, answer in mmol/L, tolerance 0.01.

Answer: 0.48 mmol/L

The

NaCl

supplies a fraction

20/50 = 0.40

of the dose. The remaining fraction

0.60

must come from

CaCl X_{2}

, giving

c_{2} = 0.60 \times 0.80 = 0.48

mmol/L.