Question

The mole percentages of \(SiO_2\), \(Al_2O_3\), and \(K_2O\) in a granitic rock are 84.21, 7.89 and 7.89, respectively. The molar proportion (in %) of K-feldspar in the rock is \(\underline{\hspace{1cm}}\) . (round off to one decimal place)

Hint:

To calculate the molar proportion of a compound in a rock, divide the mole percentage of the compound by the sum of the mole percentages of all compounds.

Answer 1

Correct Answer: 29.9 - 30.1

The molar proportion of K-feldspar can be calculated based on the molecular formula of the rock. The formula for K-feldspar is \( \text{K}_2\text{O} \cdot \text{Al}_2\text{O}_3 \cdot 6 \text{SiO}_2 \). The mole percentage of K2O is given as 7.89%. Since the molar mass of K2O, Al2O3, and SiO2 is 94.2, 101.96, and 60.08 g/mol, respectively, the molar fraction can be computed as: \[ \text{Molar Fraction of K-feldspar} = \frac{7.89}{84.21 + 7.89 + 7.89} \times 100 \approx 29.9. \] Thus, the molar proportion of K-feldspar is \( \boxed{29.9} % \).