Question

The nitrogen content of organic compounds can be determined by the Dumas method. The compound in question is first reacted by passage over hot CuO(s): Compound 8888n N21g21 CO21g21 H2O1g2 The product gas is then passed through a concentrated solution of KOH to remove the CO2. After passage through the KOH solution, the gas contains N2 and is saturated with water vapor. In a given experiment a 0.253-g sample of a compound produced 31.8 mL N2 saturated with water vapor at 258C and 726 torr. What is the mass percent of nitrogen in the compound?

Answer 1

Answer:

Mass percent of nitrogen in the compound is 13,3%

Explanation:

Dumas method is an analytical method to determine nitrogen content in samples, thus:

CₐHₓNₙ + (2a+x/2) CuO  → aCO₂ +  ˣ/₂ H₂O +  ⁿ/₂ N₂ + (2a+x/2) Cu

As the CO₂ is removed with KOH, in the mixture you have H₂O and N₂

At 25°C. the vapor pressure of water is 23,8 torr, that means that the pressure due to N₂ is:

726torr - 23,8torr = 702,2 torr.

Using gas law:

n = PV/RT

Where:

P is pressure (702,2torr≡ 0,924 atm)

V is volume (0,0318L)

R is gas constant (0,082atmL/molK)

And T is temperature (25°C≡298,15K)

Replacing, number of moles of N₂, n, are:

n = 1,20x10⁻³moles of N₂. In grams:

1,20x10⁻³moles of N₂×$\frac{28g}{1mol}$ = 0,0336 g of N₂.

Thus, mass percent of nitrogen in the compound is:

$\frac{0,0336g N}{0,253gSample}$×100= 13,3%

I hope it helps!