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Question

if a 0.25 m solution of saltwater containing 800 ml is left uncovered overnight, and 100 ml of the solvent evaporates, what is the concentration of the remaining solution?

Answer

The concentration of the remaining solution is roughly 0.29 M, and we need to determine the volume of the solution after evaporation, the amount of solute, and the concentration of the residual solution. The original solution has a concentration of 0.25 M, meaning 0.25 moles of salt per litre of solution. To calculate the amount of salt in the initial solution, we use the equation concentration times volume equals moles of solute. The initial volume is 800 ml, which converts to 0.8 L. Therefore, the initial solution contained 0.25 mol/L x 0.8 L = 0.2 mol of salt. After the solvent has evaporated, the volume of the remaining solution is 700 ml. However, the amount of salt has not changed, so the concentration can be calculated as moles of salt per volume of solution. Therefore, the residual solution has a concentration of 0.2 mol/0.7 L = 0.29 M.