Instruction

1

Mass fraction of substances is given by: w = m ()/m(cm), where w is the mass fraction of the substance m – mass of the substance, m(cm) is the weight of the mixture. If the substance dissolved, then the formula is: w = m ()/m(R-RA), where m(R-RA) is the mass of the solution. A lot of solution if necessary, can also be found: m(R-RA) = m () + m(R-La), where m(R-La) is the mass of the solvent. If desired, the mass fraction can be multiplied by 100%.

2

If the clause is not given weight, it can be calculated using several formulas to choose need the help of the data values in the condition. The first formula to find mass: m = V*p, where m – mass, V – volume, p – density. The following formula is: m = n*M, where m is the mass, n is amount of substance, M is the molar mass. Molar mass in turn is the sum of the atomic masses of the elements included in the composition of the substance.

3

For a better understanding of this material will solve the problem. A mixture of copper and magnesium sawdust weighing 1.5 g were treated with excess of sulfuric acid. As a result of reaction of the separated hydrogen with a volume of 0.56 liters (normal conditions). Calculate the mass percent of copper in the mixture.

In this task, the reaction takes place, write its equation. Of two substances with an excess of hydrochloric acid interacts only magnesium: Mg + 2HCl = MgCl2 + H2. To find the mass percent of copper in mixture, it is necessary to substitute the values in the following formula: w(Cu) = m(Cu)/m(cm). The weight of the mixture is given, find the mass of copper: m(Cu) = m(cm) – m(Mg). Looking for a lot of magnesium m(Mg) = n(Mg)*M(Mg). To find the amount of substance of magnesium will help the reaction. Find the amount of substance of hydrogen: n = V/Vm = 0,56/22,4 = 0,025 mol. The equation shows that n(H2) = n(Mg) = 0,025 mol. Calculate the mass of magnesium, knowing that the molar mass of magnesium is 24 g/mol: m(Mg) = 0,025*24 = 0.6 g. Find the mass of copper: m(Cu) = 1,5 – 0,6 = 0,9 g it Remains to calculate the mass fraction w(Cu) = 0,9/1,5 = 0,6 or 60%.

In this task, the reaction takes place, write its equation. Of two substances with an excess of hydrochloric acid interacts only magnesium: Mg + 2HCl = MgCl2 + H2. To find the mass percent of copper in mixture, it is necessary to substitute the values in the following formula: w(Cu) = m(Cu)/m(cm). The weight of the mixture is given, find the mass of copper: m(Cu) = m(cm) – m(Mg). Looking for a lot of magnesium m(Mg) = n(Mg)*M(Mg). To find the amount of substance of magnesium will help the reaction. Find the amount of substance of hydrogen: n = V/Vm = 0,56/22,4 = 0,025 mol. The equation shows that n(H2) = n(Mg) = 0,025 mol. Calculate the mass of magnesium, knowing that the molar mass of magnesium is 24 g/mol: m(Mg) = 0,025*24 = 0.6 g. Find the mass of copper: m(Cu) = 1,5 – 0,6 = 0,9 g it Remains to calculate the mass fraction w(Cu) = 0,9/1,5 = 0,6 or 60%.

Note

Mass fraction cannot be greater than unity or if it is expressed as a percentage more than 100%.