Question

Find the mass of lead $\left( {Pb} \right)$ atom. Given the molecular weight of lead is $207.2g/mol.$
A.$34.39 \times {10^{ - 19}}g.$
B.$67.04 \times {10^{ - 23}}g.$
C.$67.04 \times {10^{ - 19}}g.$
D.$34.39 \times {10^{ - 23}}g.$

Answer 1

Hint: If we want the weight in grams of lead, simply divide the molecular weight by the Avogadro’s number.
The mass of an atom can be calculated using the formula,
Mass of an atom${\text{ = }}\dfrac{{{\text{Molar}}\,{\text{Mass}}}}{{Moles}}\,g$

Complete step by step answer:
We know that the molar mass is the mass of one mole of the substance. The unit of molar mass is ${\text{g/mol}}{\text{.}}$
A mole of a substance is equal to Avogadro’s number of that substance.
Given,
The molecular weight of the substance is $207.2g/mol.$
We already have seen that one mole of a substance is equal to $6.022 \times {10^{23}}.$
$1\,mole\,of\,Pb = 6.022 \times {10^{23}}.$
Using the formula,
Mass of an atom${\text{ = }}\dfrac{{{\text{Molar}}\,{\text{Mass}}}}{{Moles}}\,g$
Mass of an atom$ = \dfrac{{207.2}}{{6.022 \times {{10}^{23}}}}g$
Mass of an atom$ = 34.40 \times {10^{ - 23}}g$
The obtained value is equal to option (D).

Additional Note:We can calculate the molecular weight by adding the mass of an atom multiplied by the number of atoms present in the molecule.
For Example, The molecular weight of the ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{6}}}$ is calculated as,
We know the atomic weight of carbon is $12$ and the atomic weight of hydrogen is $1$
Therefore, the molecular weight of the ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{6}}} = (2 \times 12) + (6 \times 1) = 30$

Note:
We must know that the atomic mass of the substance is the sum of protons or electrons in an atom. For example, the atomic mass of carbon is 6 which means the number of electrons is six.
The molecular weight of molecules is determined by various methods:
Mass spectrometry: This method is usually used in determining the mass of small molecules.
Hydrodynamic method: In this method weight is found out by Mark-Houwink relations.
Static Light scattering method: In this method molecular weight is determined from the amount of light scattered using the Zimm method.
-The reaction is also used for the industrial preparation of 3-hydroxy-2-naphthoic acid which is the starting material to azo dyes and pigments.
-We can prepare aspirin by reacting salicylic acid with acetic anhydride. Aspirin is generally used as a painkiller.