How to find the mass of a substance

The space around us is filled with different physical bodies, which consist of different substances with different masses. School courses in chemistry and physics, introducing the concept and method of finding the mass of a substance, were listened to and safely forgotten by everyone who studied at school. But meanwhile, the theoretical knowledge acquired once may be needed at the most unexpected moment.

Calculation of the mass of a substance using the specific density of a substance. Example - there is a barrel of 200 liters. You need to fill the barrel with any liquid, say, light beer. How to find the mass of a filled barrel? Using the substance density formula p=m/V, where p is the specific density of the substance, m is the mass, V is the volume occupied, it is very easy to find the mass of a full barrel:

• Measures of volumes - cubic centimeters, meters. That is, a barrel of 200 liters has a volume of 2 m³.
• A measure of specific gravity is found using tables and is a constant value for each substance. Density is measured in kg/m³, g/cm³, t/m³. The density of light beer and other alcoholic beverages can be viewed on the website. It is 1025.0 kg/m³.
• From the density formula p \u003d m / V => m \u003d p * V: m \u003d 1025.0 kg / m³ * 2 m³ \u003d 2050 kg.

A barrel of 200 liters, completely filled with light beer, will have a mass of 2050 kg.

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

• The molar mass is calculated in the unified SI measurement system 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)=M (Na)+M (I). The value of the molar mass of each element can be calculated from the table, or you can use the online calculator on the site: M (NaI) \u003d M (Na) + M (I) \u003d 23 + 127 \u003d 150 (g / mol).
• From the general formula M (NaI) \u003d m (NaI) / v (NaI) => m (NaI) \u003d v (NaI) * M (NaI) \u003d 0.6 mol * 150 g / mol \u003d 90 grams.

The mass of sodium iodide (NaI) with a mass fraction of a substance of 0.6 mol is 90 grams.

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

• Dilution of the solution with water. The mass of the dissolved X substance does not change m (X)=m'(X). The mass of the solution increases by the mass of added water m '(p) \u003d m (p) + m (H 2 O).
• Evaporation of water from solution. The mass of the solute X does not change m (X)=m' (X). The mass of the solution is reduced by the mass of evaporated water m '(p) \u003d m (p) -m (H 2 O).
• Drainage of two solutions. The masses of solutions, as well as the masses of the solute X, add up when mixed: m '' (X) \u003d m (X) + m ' (X). m '' (p) \u003d m (p) + m '(p).
• Dropout of crystals. The masses of the dissolved substance X and the solution are reduced by the mass of the precipitated crystals: m '(X) \u003d m (X) -m (precipitate), m '(p) \u003d m (p) -m (precipitate).

An algorithm for finding the mass of a reaction product (substance) if the yield of the reaction product is known. The product yield is found by the formula η = * 100%, where m (x practical) is the mass of the product x, which is obtained as a result of the practical reaction process, m (x theoretical) is the calculated mass of the substance x. Hence m (x practical)=[η*m (x theoretical)]/100% and m (x theoretical)=/η. The theoretical mass of the resulting product is always greater than the practical one, due to the reaction error, and is 100%. If the problem does not give the mass of the product obtained in a practical reaction, then it is taken as absolute and equal to 100%.

Options for finding the mass of a substance are a useful course of schooling, but methods that are quite applicable in practice. Everyone can easily find the mass of the required substance by applying the above formulas and using the proposed tables. To facilitate the task, write down all the reactions, their coefficients.