How to calculate the mass fraction of the element in the substance

The mass fraction of the element in the substance is one of the topics that is included in the chemistry course. Skills for definition of this parameter can be useful in checking the knowledge during the control and independent work, as well as on the exam in chemistry.

You will need

• - the periodic system of chemical elements D. I. Mendeleev

Instruction

1. In order to calculate the mass fraction, you must first find the relative atomic mass (Ar) of the desired element, and the relative molecular mass (Mr) of the substance. Next, apply the formula, which is determined and the mass fraction of the element (W)W = Ar (x) / Mr x 100 %, которойW - mass fraction of the element (measured in fraction or %);Ar (x) is the relative atomic mass of element;Mr is the relative molecular mass of the substance.To determine the relative atomic and molecular masses, use the periodic system of chemical elements D. I. Mendeleev. The calculations take into account the number of atoms of each element.
2. Example No. 1. Determine the mass fraction of hydrogen in water.Look at the table of D. I. Mendeleev of the relative atomic mass of vodoroda (H) = 1. As in formula 2 of the hydrogen atom, therefore, 2Ar (H) = 1 x 2 = 2Вычислите the relative molecular mass of water (H2O), which consists of 2 Ar (H) and 1 Ar (O).Mr (H2O) = 2Ar (H) + Ar (O)Ar (O) = 16, sledovatelno (H2O) = 1 x 2 + 16 = 18
3. Write down a General formula for determining the mass fraction of the element W = Ar (x) / Mr x 100% Now, write down a formula with respect to the condition задачиW (H) = 2 Ar (H) / Mr (H2O) x 100 %Make вычисленияW (H) = 2 / 18 x 100 % = 11,1 %
4. Example No. 2. Determine the mass fraction of oxygen in the copper sulfate (CuSO4).Look at the table of D. I. Mendeleev of the relative atomic mass of kisloroda (O) = 16. As in formula 4 oxygen atoms, therefore 4 Ar (O) = 4 x 16 = 64Вычислите the relative molecular mass of copper sulphate (CuSO4), which consists of 1 Ar (Cu), 1 Ar (S) and 4 Ar (O).Mr (CuSO4) = Ar (Cu) + Ar (S) + 4 Ar (O).Ar (Cu) = 64, Ar (S) = 324 Ar (O) = 4 x 16 = 64, sledovatelno (CuSO4) = 64 + 32 + 64 = 160
5. Write down a General formula for determining the mass fraction of the element W = Ar (x) / Mr x 100% Now, write down a formula with respect to the condition задачиW (A) = 4 Ar (O) / Mr (CuSO4) x 100 %Make вычисленияW (On) = 64 / 160 x 100 % = 40 %