If this the problem the volume and density calculate the mass as follows: m = V*p, where m – mass, V – volume, p – density.
In other cases calculate the mass: m = n*M, where m is the mass, n is amount of substance, M is the molar mass. The molar mass is not difficult to calculate for this you need to add up all the atomic masses of simple substances into the complex (atomic mass listed in the table of D. I. Mendeleev on the designation of the element).
Express the mass value from the formula the mass fraction of the substance: w = m(x)*100%/m, where w is the mass fraction of the substance, m(x) is the mass of the substance, m is the mass of the solution, in which dissolved the substance. To find the mass of the substance is required: m(x) = w*m/100%.
From the formula of the product yield calculates the desired weight yield of product = mp(x)*100%/m(x), where mp(x) is the mass of the product x, obtained in the process, m(x) is the calculated mass of the substance x. Output: mp(x ) = product yield* m(x)/100% or m(x) = mp(x)*100%/ the output of the product. When given in the problem statement yield this formula will be needed. If the output of the product is not given, assume that it is equal to 100%.
If the condition is present in the reaction equation, then solve the problem on it. To do this, first make the reaction equation, then calculate from it the amount of substance received or expended for this reaction and already is the amount of substance put in the right formula. For example, Na2SO4 + BaCl2 = BaSO4 + 2NaCl. It is known that the mass of BaCl2 is equal to 10.4 g, you need to find the mass of NaCl. Calculate the amount of substance of chloride of barium, n = m/M. M (BaCl2) = 208 g/mol. n(BaCl2) = 10,4/208 = 0.05 mol. From the reaction equations, it follows that 1 mole of BaCl2 was formed 2 mol NaCl. Calculate what quantity of matter is formed from 0.05 mol BaCl2. n(NaCl) = 0,05*2/1 = 0.1 mol. In the task they needed to find the mass of sodium chloride, find pre-calculating the molar mass of sodium chloride. M(NaCl) = 23+35,5 = 58,5 g/mol. m(NaCl) = 0,1*58,5 = 5,85 g. the Problem is solved.
Units of mass can be milligrams, grams, kilograms.