## Calculate the number of molecules of sulphur (S 8 ) present in 16 g of solid sulphur.

**
Answer
**

We know that,

**
Number of moles of a substance = Given mass of the substance / Molar mass of the substance
**

**
n = m/M
**
----- (1)

Also,

**
Number of moles of a substance = Number of particles of the substance / Avogadro's Constant
**

n = N / N
_{
O
}

**
n = N / 6.022 x 10
**
**
23
**
------- (2)

From the above 2 formulae, we can say that,

**
n = m / M = N / 6.022 x 10
**
**
23
**
---- (3)

Given mass of sulphur molecule(S
_{
8
}
) = m = 16g

Molar mass of Sulphur Molecule (S
_{
8
}
) = M = 256g

We need to find Number of molecules, ie, N

Putting values in (3), we get

16 / 256 = N / 6.022 x 10 23

**
N = 6.022 x 10
**
**
23
**
**
x (16 / 256)
**

= 6.022 x 10 23 / 16

= 3.011 x 10 23 /8

= 0.37 x 10
23
molecules of Sulphur(S
_{
8
}
)

=
**
3.7 x 10
**
**
22
**
**
molecules of Sulphur (S
**
_{
8
}
**
)
**