Question

NCERT Based
Number of atoms present in 120 a.m.u. of Ca is
[NCERT Pg. 16
1.
(1) 3 NA
(2) 6 NA
(3) 3
(4) 6​

Answer 1

Answer:

(3) 3Na

Explanation:

Mass of 1 calcium atom=40 a.m.u or 40u

Therefore, number of atoms in 120 a.m.u. will be=120

$\frac{120}{40} = 3 \: atoms \:$

If they asked number of atoms in atoms in 120 grams of Ca,

Then

$number \: of \: moles = \frac{given \: mass}{molecular \: mass}$

$\frac{120g}{40g} = 3moles$

$number \: of \: moles = \frac{number \: of \: atoms}{na}$

$number \: of \: atoms = na \times moles$

Therefore the atoms present in 120 grams of Ca is equal to3Na.

Hope it helps mate!

Answer 2

Given: 120 amu of Ca.

To find: We have to find the number of atoms.

Solution:

To determine the number of atoms in 120amu of Ca we have to follow the below steps-

One mole of Ca has an atomic weight of 40amu.

40amu of Ca has $N_A$ number of atoms.

Where $N_A$ is called Avagrado's number.

120amu of Ca will have-

$\frac{120}{40} \times N_A = 3N_A$

The result is 3$N_A$.

Thus, 120amu of Ca has 3$N_A$ atoms.

The correct option is 1.