To coagulate 20.0 mL of a gold sol, 5.0 mL of 0.50 M $NaCl$ solution is needed. Flocculation value is the number of millimoles of electrolyte per litre of sol (volume of the sol only, ignoring the added electrolyte volume). Find the flocculation value of $NaCl$ for this sol (nearest integer).

Numerical answer type, answer in mmol/L, tolerance 0.5.

Answer: 125 mmol/L

Moles of NaCl

= 5.0 \times 0.50 = 2.5

mmol. Sol volume is

20.0 mL = 0.0200 L

. Flocculation value

= 2.5/0.0200 = 125 mmol/L