# Units of measurement in chemistry

After studying the article, you will learn how much atoms weigh, how to calculate the number of atoms by the mass of matter and how to read the label to find out how much substance is in the solution

## How much does an atom weigh?

The atomic unit of mass (AU) [u] is a unit of mass measurement used in chemistry to express the mass of atoms,
for example, a hydrogen atom (_{1}H) weighs 1,00794u, a vanadium atom (_{23}V) weighs 50.9415u, and so on.

1 u ≈ 1.660 5402(10) × 10 −27 kg.

It is not very convenient to use daltons in calculations, so we introduced the relative mass of the atom - **atomic mass**.
Atomic mass is the ratio of the mass of an atom to ^{1}⁄_{12} of the mass of a carbon atom and the value is dimensionless.
The atomic mass for each element is recorded in the periodic table.

## How many atoms are there in a gram of matter?

One gram of chlorine contains 93.03 × 10^{23} molecules, again for the convenience of calculations was
the unit of measurement "mole" has been introduced. A mole is 6,022 140,857(74) × 10^{23} units. In moles it is possible to measure that
For example, 1 mole of people is 6,022 140 857(74) × 10^{23} people. This number is called the Avogadro number,
it was calculated as the number of atoms in 12 grams of carbon.

Since a.e.m. and mol were calculated based on the mass of carbon, the following ratios should be remembered:

- 1 au =
^{1}⁄_{12}carbon masses in grams - 1 mol = number of atoms in 12 grams of carbon

How can these ratios be used? If we take the atomic mass of any element, the gram/mol ratio will be numerically equal to this number: the mass of sulfur is 32.066 AU, so one mole of sulfur weighs 32.066 grams.

## Concentration

Concentration is the amount/mass/volume of a substance dissolved in water or another solution.
Various ratios are used for calculations: g / mol, mol/ l, mol/kg and others. The most popular are ppm (parts per
million), molarity - mol/kg and molarity - mol/L. It is necessary to remember that concentration is considered
as the fraction/mass/volume of the solute to the fraction/mass/volume __ of the total__ solution. For example, "0.9% sodium chloride solution"
means that 9 grams of sodium chloride were stirred in 991 grams of water. "Hydrogen peroxide solution 3%" (peroxide
hydrogen) means that 3 grams of hydrogen peroxide is dissolved in 97 grams of water.