$200 mL$ of $0.30 M$ $BaCl X_{2}$ is mixed with $300 mL$ of $0.25 M$ $Na X_{2} SO X_{4}$ and the insoluble $BaSO X_{4}$ is formed completely (volumes additive, ignore solubility). Find the concentration of $SO X_{4} X^{2 -}$ remaining in solution, in mmol/L, to one decimal place.

Numerical answer type, answer in mmol/L, tolerance 0.2.

Answer: 30 mmol/L

Ba X^{2 +}

= 0.200 x 0.30 = 0.06 mol;

SO X_{4} X^{2 -}

= 0.300 x 0.25 = 0.075 mol.

Ba X^{2 +}

is limiting, so

SO X_{4} X^{2 -}

left = 0.015 mol in 0.500 L = 0.03 M = 30.0 mmol/L.