Question

Calculate the number of moles of helium present in $6.46g$ .(the atomic weight of helium is $4\,a.m.u$ )
A. $16.15moles$
B. $1.615moles$
C. $161.5moles$
D. $0.1615moles$

Answer 1

One mole is the amount of substance that contains as many particles as that are present exactly in $12g$of carbon isotope $^{12}C$ . $amu$ stands for atomic mass unit. One mole of any substance contains Avogadro’ number of particles.

Complete step by step answer:
$amu$ is an atomic mass unit. One atomic mass unit is defined as a mass exactly equal to one-twelfth of the mass of one carbon$- 12$ atom . At present $amu$ has been replaced by $u$ which is unified mass.
As we have defined mole, we can also define the mass of one mole of any substance. The mass of one mole of a substance when expressed in grams is called its molar mass. The molar mass in grams is numerically equal to atomic mass unit or unified mass.
Thus we can say that for example,
Molar mass of water $= 18gmo{l^{ - 1}} = 1\,mole\,of\,{H_2}O$ .
Similarly in case of helium the atomic weight of helium in $amu$ is numerically equal to molar mass in grams or is equal to the mass of one mole of helium.
$Molar\,mass\,of\,He = 4gmo{l^{ - 1}} = 1\,mole\,of\,He$
So number of moles in $6.46g$ of helium $= \dfrac{{1 \times 6.46}}{4} = 1.615moles$
So the number of moles in $6.46g$ of helium is $1.615moles$

So, the correct answer is Option B.

Note: One mole of any substance has Avogadro’s number of particles or entities. It is also called Avogadro's constant. The value of avogadro’s number is $6.023 \times {10^{23}}$ . It is denoted as ${N_A}$ .
Thus one mole of water molecules $= 6.023 \times {10^{23}}$ molecules of water $= 18gmo{l^{ - 1}}\,{H_2}O$ .
Molar mass is expressed in $gmo{l^{ - 1}}$ .
Mass of one mole of any substance is equal to the molar mass.