Percent by mass is one way of expressing concentration in terms of the mass of the solute relative to the total mass of the solution. Calculation for the amount of the stock solution needed is as follows:
C1M1 = C2M2
where C is the concentration in percent by mass and M is the mass of the solution
.87 x M1 = .355 x 200
M1 = 81.61 g of the stock solution is needed.
