Water - plain and at the same time irreplaceable substance in the nature, is considered universal solvent which is a basis for many solutions. For their correct preparation with the set concentration it is necessary to be able to calculate the mass of water.
It is required to you
- - D.I. Mendeleyev's table;
- - calculator.
Instruction
1. Example No. 1. Calculate the mass of water if the mass of the dissolved substance – chloride of sodium (NaCl) is 20 g, and the mass fraction (NaCl) is equal to 10%. For the decision it is necessary to use a formula which is applied to calculations for the subject "Solutions". W = m (the dissolved substance) / m (solution) x 100%W – a mass fraction of the dissolved substance, a %vyvedita from this formula m (solution). Therefore: m (solution) = m (the dissolved substance) / W x 100%podstavte value which are given on a statement of the problem: m (NaCl solution) = 20 g / 10% x 100% = the 200th glyuby solution consists of the dissolved substance and water. Therefore, having the mass of all solution (200 g) and mass of the dissolved substance (10 g) it is possible to calculate the mass of water: m (water) = m (solution) - m (the dissolved substance) of m (water) = 200 - 10 = 190 gotvt: m (water) = 190 g
2. Sometimes in a condition not the mass of substance, but volume can be given. In this case the formula which unites such parameters as weight, density and volume is required. Example No. 2. Calculate the mass of water if it is known that its volume of 500 ml, and density of 1 g/ml. To remember and to correctly use the formula connecting the weight, density and volume it is enough to apply simple method. If at density g/ml unit of measure, therefore, that it is necessary to find it weight (g) to divide.r = m/VTeper from this formula bring the weight which needs to be calculated to the volume (ml). Therefore: m = r x VPodstavte specified in a value condition: = 1 g/ml x 500 ml = 500 gotvt m (water): m (water) = 500 g
3. Example No. 3. Calculate the mass of water if it is known that amount of substance of water of 3 mol. For the solution of a task at first calculate the molar mass of water which will coincide with relative molecular weight in number. It is possible to make it, using the Periodic Table of D.I. Mendeleyev. Mr (H2O) = 2Ar (H) + Ar (O). 2Ar (H) = 1 x 2 = 2Ar (O) = 16Mr (H2O) = 2 + 16 = 18 M (H2O) = 18 x 1 g/mol = 18 g / moldalee use a formula which connects amount of substance (n), the weight (m) and molar mass (M): n = m / Mvyvedite from it weight: m = n x MPodstavte the values specified in a condition: = 3 mol x 18 g/mol = 54 gotvt m (water): m (water) = 54 g