Precision : It is the closeness of various measurements for the same quantity .

Accuracy : It is the agreement of a particular value to the true value of the result.

Statement : Matter can neither be created nor destroyed.

Statement : If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers. This law was given by Dalton in 1803.

e.g Hydrogen +oxygen → water

`\ \ \ \ \ \ \ 2g \ \ \ \ \ \ \ 16g \ \ \ \ \ \ \18g`


Hydrogen + Oxygen → Hydrogen peroxide

`2g \ \ \ \ \ \ \ \ \ 32 g \ \ \ \ \ \ \ \ \ \ \ \ 34g`


Here, the ratio of masses of oxygen (i.e. `16` g and `32` g) which combine with a fixed mass of hydrogen (`2` g) is `1 : 2`.

Statement : When gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at
same temperature and pressure.

Hydrogen + Oxygen `→` Water

`\ \ \ \ 100 \ mL \ \ \ \ \ \ \ 50 \ mL \ \ \ \ \ \ \100 \ mL`

Here, ratio of volumes of hydrogen and oxygen is `2:1`.

This law is also known as the law of definite proportions by volume.

Statement : Equal volumes of gases at the same temperature and pressure should contain equal number of molecules.

Percentage composition gives the idea about the purity of a given sample by analyzing the given data.

Mass `%` of an element `= text(mass of that element in the compound)/text(molar mass of the compound)`

e.g. For `H_2O`

Molar mass of water = `18.02`g

Mass `%` of `H` `= (2xx1.008)/18.02 xx100`

Mass `%` of `O = 16.00/18.02 xx100= 88.79`