Question

An unknown amount of mercury (II) oxide was decomposed in the lab. Mercury metal was formed and 5.20 L of oxygen was released at a pressure of 0.970 atm and 390.0 K. What was the initial mass of mercury oxide in the sample?

Answer 1

Answer:

• 68.3g

Explanation:

1. Word equation:

• mercury(II) oxide → mercury + oxygen

2. Balanced molecular equation:

• 2HgO → 2Hg + O₂(g)

3. Mole ratio

Write the ratio of the coefficients of the substances that are object of the problem:

       $2molHgO/1molO_2$

4. Calculate the number of moles of O₂(g)

Use the equation for ideal gases:

          $pV=nRT\\\\\\n=\dfrac{pV}{RT}\\\\\\n=\dfrac{0.970atm\times5.20L}{0.08206atm.L/K.mol\times 390.0K}\\\\\\n=0.1576mol$

5. Calculate the number of moles of HgO

         $\dfrac{2molHgO}{1molO_2}\times 0.1576molO_2=0.315molHgO$

6. Convert to mass

• mass = # moles × molar mass

• molar mass of HgO: 216.591g/mol

• mass = 0.315mol × 216.591g/mol = 68.3g