The relative molecular mass of substance shows how much the molecule of this substance is heavier 1/12 atoms of pure carbon. It can be found in case its chemical formula is known, using the periodic table of elements of Mendeleyev. Otherwise, use other methods of finding of molecular weight, considering that it is in number equal to the molar mass of substance expressed in grams on mol.
It is required to you
- - periodic table of chemical elements;
- - tight cylinder;
- - scales;
- - manometer;
- - thermometer.
Instruction
1. If the chemical formula of substance is known determine its molecular weight, using the periodic table of chemical elements of Mendeleyev. For this purpose define elements which enter a substance formula. Then, find their relative atomic masses which are written down in the table. If atomic mass is presented in the table by fractional number, round it to the next whole. If chemical formula contains several atoms of this element, increase the mass of one atom by their quantity. Put the received atomic masses and receive the relative molecular mass of substance.
2. For example, to find the molecular mass of sulfuric H2SO4 acid, find relative atomic masses of elements which enter a formula, according to hydrogen, sulfur and Ar (H) =1, Ar (S) =32, Ar (O) =16 oxygen. Considering that hydrogen in a molecule 2 atom, and oxygen 4 atoms, calculate the molecular mass of the substance Mr(H2SO4) of =2•1+32+4∙16=98 atomic units of mass.
3. In case the amount of substance in moths ν is known and the mass of substance m expressed in grams, determine its molar weight for this purpose weight divide into amount of the substance M=m/ν. It will be in number equal to its relative molecular weight.
4. If the quantity of molecules of substance N, the known mass of m is known, find its molar weight. It will be equal to molecular weight, having found the weight relation in grams to quantity of molecules of substance in this weight, and increase result by constant Avogadro of NA =6,022^23 1/mol (M=m∙N/NA).
5. To find the molecular mass of unknown gas, find its weight in a tight cylinder of the known volume. For this purpose pump out gas from it, having created a vacuum there. Weigh a cylinder. Then pump gas back and again find its weight. The difference of mass of the empty and downloaded cylinder will also be equal to the mass of gas. Measure pressure in a cylinder by means of the manometer in Pascals, and temperature in Kelvins. For this purpose take ambient temperature, it will be equal to temperature in a cylinder in degrees Celsius to transfer it to Kelvins, add to the received value 273. Determine the molar mass of gas, having found the work of temperature of T, mass of gas m and a universal gas constant R (8.31). Divide the received number into values of pressure of P and volume of V measured in m³ (M=m•8.31•T/(P•V)). This number will correspond to the molecular mass of the studied gas.