Question

Calculate the number of moles for the following
i) $52g$ of He (finding mole from mass).
ii) $12.044\times {{10}^{23}}$ number of He atoms (finding mole from number of particles).

Answer 1

We should know the formula to calculate number of moles

$$\text{number of moles n = }\dfrac{\text{given weight of the element (w) }}{\text{molar mass of the element (M)}}$$ (Finding mole from mass) $$\text{number of moles n = }\dfrac{\text{number of particles}}{\text{Avogadro number of particles (}{{\text{N}}_{\text{A}}}\text{)}}$$ (Finding moles from number of particles) **Complete step by step answer:** i) Given mass of helium ($$He$$), w = 52 g Atomic weight of helium = 4 units Molar mass of helium, M = 4 g $$\begin{aligned} & \text{number of moles n = }\frac{\text{given weight of the element (w) }}{\text{molar mass of the element (M)}} \\\ & \text{ =}\dfrac{\text{52g}}{\text{4g}} \\\ & \text{ =13moles} \\\ \end{aligned}$$ (Finding moles from mass) ii) In the question the given number of He atoms =$$12.044\times {{10}^{23}}$$ Avogadro number, NA = $$6.022\times {{10}^{23}}$$particles = 1 mole $$\begin{aligned} & \text{number of moles n = }\frac{\text{number of particles}}{\text{Avogadro number of particles (}{{\text{N}}_{\text{A}}}\text{)}} \\\ & \text{ =}\dfrac{\text{12}\text{.044 }\\!\\!\times\\!\\!\text{ 1}{{\text{0}}^{\text{23}}}}{\text{6}\text{.022 }\\!\\!\times\\!\\!\text{ 1}{{\text{0}}^{\text{23}}}} \\\ & \text{ =2moles} \\\ \end{aligned}$$ (Finding moles from number of particles) **Additional Information:** We cannot calculate the number of moles of a particular atom without knowing about the number of particles present or mass of the chemical element present. Make sure that we should use Avogadro number while calculating the number of moles from the number of particles and molar mass of a particular atom of chemical while calculating the number of moles from mass of the atom or chemical. **Note:** Don’t be confused with the formula to calculate the number moles from molar mass and number moles from number of particles. In the calculation of number of moles from number of particles we should use Avogadro number and in the calculation of number of moles from mass we should use molar mass of the element.