The international system of units (SI)
Quantity used frequently in chemistry
Mathematical operation using scientific notation
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.
Rules for determining the number of significant figures
Mathematical operations of significant figures
Rules for rounding off the numbers
Law of conservation of mass
Statement : Matter can neither be created nor destroyed.
Law of definite proportions
Law of multiple proportions
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`.
Gay Lussac's law of gaseous volume
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`
Empirical formula and molecular formula