There are two variants of formulation of the problem: 1) when it is necessary to determine the mass element share in veschestve- 2) when it is necessary to determine the mass fraction of the solute.
You will need
- It is necessary to determine which option relatesyour task. In the case of the first option, you will need periodic table. In the case of the second - it is necessary to know that the solution consists of two components: the solute and solvent. And the weight of the solution is equal to the masses of the two components.
In the case of the first variant of the problem:
In the periodic table we find the molar mass of the substance. Molar mass is the sum of the atomic weights of the elements that make up matter.
For example, the molar mass (Mr) of calcium gidrokisda Ca (OH) 2: Mr (Ca (OH) 2) = Ar (Ca) + (Ar (O) + Ar (H)) * 2 = 40 + (16 + 1) * 2 = 74.
The molar mass of the atoms are taken from the periodic table.
We calculate the mass fraction of the element (?), Such as calcium hydroxide to calcium.
Mass fraction is the ratio of the atomic weight of an element to the molar mass of the substance? = Ar: Mr.
In the case of calcium hydroxide:? (Ca) = 0.54 = 40:74. It is the mass fraction of the element as a decimal.
In the case of the second variant of the problem:
Determine what weight you are, namely, weight of the solute and mass of solution or weight of the solute and solvent mass.
If a given weight of the solute and solution, the mass fraction is the ratio of the mass of dissolved (RV) of the substance to the weight of the solution (solution).
? = M (RV.): M (r-ra)
? For example, if the mass of 40 g salt, and 100 g of the solution weight, the (salt) = 40: 100 = 0.4. It is the mass fraction of the solute in unit shares.
If a given mass of solute and solvent, it is first necessary to determine the weight of the solution. The weight of the solution (solution) is equal to the sum of the weight of the solute (RV) and the solvent (p-la).
m (r-ra) = m (RV.) + m (p-la)
For example, if the mass of salt 40 g and 60 g of water weight, then m (solution) = 40 + 60 = 100 (g).
It then calculates the mass fraction of the solute is similar to the previous step.
To find the mass fraction of a percentage you multiply the mass fraction expressed as a decimal 100.
? (Ca) = 0.54 * 100 = 54%