Mass fraction is the ratio of the mass of a substance to the mass of the solution or mixture: w = m ()/m(R-RA), where w – mass fraction, m(in) is the mass of substance, m(R-RA) – mass of solution, or w = m ()/m(cm), where m(cm) is the weight of the mixture. Expressed in fraction or percentage.
Additional formulas which can be necessary for solving problems on the mass fraction of the substance:
1)m = V*p, where m – mass, V – volume, p – density.
2)m = n*M, where m is the mass, n is the number of the substance, M is the molar mass.
The mole fraction is the ratio of the number of moles of a substance to number of moles of all substances: q = n(q)/n(total), where q is the mole fraction, n(q) – the quantity of a specific substance, n(total) is the total number of substances.
Additional formulas:
1)n = V/Vm, where n – amount of substanceV – volume, Vm – molar volume(at normal conditions is equal to 22.4 l/mol).
2)n = n/Na where n is the number of the substance, N is the number of molecules, Na – the Avogadro constant(is constant and equal to 6,02*10 23 1/mol).
The volume fraction is the ratio of the volume of the substance to the volume of the mixture: q = V(q)/V(cm), where q is the volume fraction, V(in) – volume of the substance, V(cm) is the volume of the mixture.
Molar concentration – the ratio of the substance to the volume of the mixture: Cm = n(V)/V(cm), where Cm is the molar concentration(mol/l), n – amount of substance(mol) V(cm) – volume of mixture(l). Solve the problem by the molar concentration. Determine the molar concentration of a solution obtained by dissolving sodium sulfate with a mass of 42.6 g in water weight of 300 g when the density of the resulting solution is equal to 1.12 g/ml. Write the formula for molar concentration: Cm = n(Na2SO4)/V(cm). See that it is necessary to find the amount of substance of sodium and the volume of the solution.
Calculated: n(Na2SO4) = m(Na2SO4)/M(Na2SO4).
M(Na2SO4) = 23*2+32+16*4 = 142 g/mol.
n(Na2SO4) = 42,6/142 = 0,3 mol.
Looking for a solution volume: V = m/p
m = m(Na2SO4) + m(H2O) = 42,6 + 300 = 342,6 G.
V = 342,6/1,12 = 306 ml = 0,306 L.
Substitute in the General formula: Cm = 0,3/0,306 = 0.98 mol/L. the Problem is solved.