The mass of substance is required to be found in many tasks. It can be done by means of special formulas. Usually at a statement of the problem there is a reaction by means of which there are some sizes.
Instruction
1. At the volume given in a statement of the problem and density calculate weight as follows: m = V*p where m is weight, V – the volume, p – density.
2. In other cases count weight so: m = n*M where m is weight, n is amount of substance, M – molar weight. The molar mass not difficultof to count, for this purpose it is necessary to put all atomic masses of the simple substances which are a part difficult (atomic masses are specified in D.I. Mendeleyev's table on designation of an element).
3. Express value of weight from a formula of a mass fraction of substance: w = m (x) *100% / m where w is a mass fraction of substance, m(x) is the mass of substance, m is the mass of solution in which this substance is dissolved. To find the mass of substance it is necessary: m(x) = % w*m/100.
4. From a formula of a product yield calculate the weight necessary to you: the product yield = mp(x) of *100% / m(x) where mp(x) is the mass of product x received in real process, m(x) is the calculated mass of substance x. You remove: mp(x) = product yield * m (x)/100% or m(x) = mp(x)*100%/product yield. At the product yield given in a statement of the problem this formula will be necessary. If the product yield is not given, then it is necessary to consider that it is equal to 100%.
5. If at a condition there is a reaction equation, then solve a problem of it. For this purpose at first work out the reaction equation, then calculate from it the amount of the substance received or spent for this reaction and already substitute this amount of substance in the necessary formulas. For example, Na2SO4 + BaCl2 = BaSO4 + 2NaCl. It is known that the mass of BaCl2 is equal to 10.4 g, it is necessary to find the mass of NaCl. Calculate amount of substance of chloride of barium: n = m/M. M (BaCl2) = 208 g/mol. n(BaCl2) = 10.4/208 = 0.05 mol. Follows from the equation of reaction that from 1 asking BaCl2 was formed by 2 mol of NaCl. Calculate what amount of substance it is formed of 0.05 mol of BaCl2. n(NaCl) = 0.05*2/1 = 0.1 mol. It was required to find the mass of chloride of sodium in a task, find, previously having calculated the molar mass of chloride of sodium. M (NaCl) = 23+35.5 = 58.5 g/mol. m(NaCl) = 0.1*58.5 = 5.85 g. The task is solved.