Chemistry Gases MDCAT 2018
PMDC Verified Question 57 of 65
Which of the following is the correct equation to calculate relative molecular mass of a gas?
A
\( M = \frac{mPRT}{V} \)
B
\( M = \frac{mPR}{VT} \)
C
\( M = \frac{PV}{mRT} \)
D
\( M = \frac{mRT}{PV} \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: \( M = \frac{mRT}{PV} \)
Concept:

The relative molecular mass (M) of an ideal gas can be derived directly by substituting the definition of moles (n) into the ideal gas equation.

Formula:

$$ PV = nRT \quad \text{and} \quad n = \frac{m}{M} $$

Solution:

  • Substitute the formula for moles into the ideal gas law: \( PV = \left(\frac{m}{M}\right)RT \).


  • Rearrange the equation to solve for Molar Mass (M).


  • Multiply both sides by M: \( M \times PV = mRT \).


  • Divide both sides by PV: \( M = \frac{mRT}{PV} \).


Why other options are incorrect:

  • Options A, B, & C: These are mathematically incorrect rearrangements of the ideal gas law.

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