How to find a mass of substances

The space around us is filled with different physical bodies, which consist of different substances with different masses. School courses of chemistry and physics, informing the concept and method of finding a mass of matter, listened and safely forgotten everything that went to school. But meanwhile the theoretical knowledge acquired by once may be needed at the most unexpected moment.

Calculating the mass of the substance using the specific density of the substance. An example - there is a barrel for 200 liters. You need to fill the barrel of any liquid, say, light beer. How to find a lot of filled barrels? Using the density formula of the substance P \u003d M / V, where P is the specific density of the substance, M - mass, V - occupied volume, to find a lot of full barrel is very simple:

Volumes of volume - cubic centimeters, meters. That is, the barrel for 200 liters has a volume of 2 m³.
The measure of specific density is with the help of tables and is a constant value for each substance. The density is measured in kg / m³, g / cm³, t / m³. The density of the beer of light and other alcoholic beverages can be viewed on website. It is 1025.0 kg / m³.
From the density formula p \u003d m / v \u003d\u003e m \u003d p * v: m³ \u003d 1025.0 kg / m³ * 2 m³ \u003d 2050 kg.

Barrel volume of 200 liters, fully filled with light beer, will have a lot of 2050 kg.

Finding a mass of substance with the help of molar mass. M (x) \u003d m (x) / v (x) is the ratio of the mass of the substance to its amount, where m (x) is the molar mass of x, m (x) - the mass X, V (X) - the amount of substance X . If only 1, a known parameter is prescribed in the problem of the problem - the molar mass of the given substance, then the mass of this substance is not difficult. For example, it is necessary to find a mass of sodium iodide NAI with a substance of 0.6 mol.

The molar mass is calculated in a single measurement system S and is measured in kg / mol, g / mol. The molar mass of sodium iodide is the sum of the molar masses of each element: M (nai) \u003d m (na) + m (i). The value of the molar mass of each element can be calculated on the table, and you can with the help of an online calculator on website: M (NAI) \u003d M (Na) + m (i) \u003d 23 + 127 \u003d 150 (g / mol).
Of the general formula M (NAI) \u003d M (NAI) / V (NAI) \u003d\u003e M (NAI) \u003d V (NAI) * M (NAI) \u003d 0.6 mol * 150 g / mol \u003d 90 grams.

Sodium iodide mass (NAI) with a mass fraction of a substance of 0.6 mol is 90 grams.

Finding a mass of a substance by its mass fraction in solution. The formula of the mass fraction of the substance ω \u003d [m (substances) / m (solution)] * 100%, where ω is a mass fraction of a substance, and M (substances) and M (solution) are masses measured in grams, kilograms. The total proportion of the solution is always taken for 100%, otherwise there will be errors in the calculation. It is easy to derive from the formula of the mass fraction of the substance to derive the mass formula of the substance: M (substances) \u003d [ω * m (solution)] / 100%. However, there are some features of the change in the composition of the solution, which must be taken into account when solving the tasks on this topic:

Dilution with water solution. The mass of the dissolved X substance does not change m (x) \u003d m '(x). The mass of the solution increases on the mass of the added water M '(p) \u003d m (p) + m (H ₂O).
Water evaporation from solution. The mass of the dissolved substance X does not change m (x) \u003d m '(x). The mass of the solution decreases to the mass of the evaporation of the water M '(p) \u003d m (p) -m (H ₂O).
Digger two solutions. Mass solutions, as well as the mass of the dissolved substance X, the mixture is folded: M '' (x) \u003d m (x) + m '(x). m '' (p) \u003d m (p) + m '(P).
Loss of crystals. The masses of the dissolved substance X and the solution decrease on the mass of the crystals dropped: M '(x) \u003d m (x) -m (precipitate), M' (p) \u003d m (p) -m (precipitate).

The algorithm for finding the mass of the product of the reaction (substance), if the yield of the reaction product is known. The product output is located according to the formula η \u003d [M (x practical) / m (x theoretical)] * 100%, where M (x practical) is the mass of the product X, which was obtained as a result of the practical reaction process, M (x theoretical) - calculated Mass of substance x. From here M (x practical) \u003d [η * m (x theoretical)] / 100% and m (x theoretical) \u003d [M (x practical) * 100%] / η. The theoretical mass of the product obtained is always more practical, due to the error of the reaction, and is 100%. If the problem does not give the mass of the product obtained in a practical reaction, it means that it is taken as absolute and equal to 100%.

Options Finding a mass of a substance - a disconnected course of school learning, and the methods fully applied in practice. Everyone can easily find a mass of the required substance, applying the above formulas and using the proposed tables. To facilitate the task, prescribe all reactions, their coefficients.