Question

How do you find the formula units in 2.6 g of lithium bromide?

Answer 1

Lithium bromide is the ionic compound formed by lithium ion and bromide ions in the same proportion. The formula unit in 2.6 g of lithium bromide is calculated by dividing the mass by the molecular weight multiplied by the Avagadro’s number.

Complete step by step answer:
Given
The mass of lithium bromide is 2.6 g.
The formula unit is defined as the chemical formula with the same number of elements having the same proportion of ions as the element.
The number of formula units is calculated by the formula as shown below.
$F = \dfrac{m}{M} \times {N_A}$
Where,
F is the formula unit
m is the mass of the compound
M is the molecular weight of the compound.
${N_A}$ is the avogadro's number
The molecular weight is defined as the sum of the atomic weight of the atoms present in the compound where the number of atoms (if present) is multiplied with the atomic number.
The chemical formula of lithium bromide is LiBr which is formed from lithium ion and bromide ion.
The atomic weight of lithium is 6.941
The atomic weight of bromine is 79.904
The molecular weight of LiBr is calculated as shown below.
$\Rightarrow Molecular\;weight = 6.941 + 79.904$
$\Rightarrow Molecular\;weight = 86.845g/mol$
The Avagadro’s number is the unit of one mole of substance which is equal to $6.022 \times {10^{23}}$.
To calculate the formula unit, substitute the values in the given formula.
$\Rightarrow F = \dfrac{{2.6g}}{{86.845gmo{l^{ - 1}}}} \times 6.022 \times {10^{23}}mo{l^{ - 1}}$
$\Rightarrow F = 1.80 \times {10^{22}} \approx 2 \times {10^{22}}$
All the units are cancelled out, so the obtained value does not have any unit.
Therefore, the formula unit in 2.6 g lithium bromide is $2 \times {10^{22}}$.

Note:
Don’t get confused as the formula unit mass is also used where the atomic masses of the atoms present in the compound is added. The given formula can be also represented as $F = n \times {N_A}$ where n is the number of moles as the number of moles is calculated by dividing mass by molecular weight.