Question

Given, 3.49 g of ammonia at STP occupies volume of 4.48 $d{{m}^{3}}$. Calculate molar mass of ammonia.

Answer 1

Hint: Try to understand the question and the terms used to avoid confusion. Understand the conditions that are present when STP is mentioned. The conditions in this question means Pressure, Temperature etc. Convert all units to their respective SI units.

Complete answer:
We will try to define what STP is to gain a clear understanding of the conditions present.
STP stands for Standard Temperature and Pressure.
-Standard Temperature is ${{0}^{0}}C$ or 273.15 K.
-Standard Pressure is 1 atm or 76 cm of Hg or 101.325 kPa
To avoid the interference of Temperature and pressure in calculation reactions are performed at STP.
We have studied in previous classes that 1 mole of any substance occupies 22.4 L of volume at STP. This is also referred to as standard molar volume.
1 mole of ammonia occupies 22.4 L, then x mole of ammonia occupies 4.48 $d{{m}^{3}}$,
1 $d{{m}^{3}}$ = 1 L
$x\text{ = }\dfrac{4.48}{22.4}$
$x\text{ = 0}\text{.2 mol}$
It is given to us that 3.49 g of ammonia is taken into consideration. This means that 3.49 g of ammonia is 0.2 moles.
Molar mass of ammonia = $\dfrac{3.49}{0.2}$ = 17.45 g
Therefore, the molar mass of ammonia is 17.45 g.

Note:
NTP stands for normal temperature and pressure. NTP is used as an alternative to STP for calculation. At NTP,
- Temperature is taken as ${{20}^{0}}C$ or 293.15 K,
- Pressure is taken as 1atm or 101.325 kPa.