Question

q1. choose the correct answer for each of the following:
a) the mass in grams of any pure substance is called its __
a. atomic b. molecular c. formula d. molar
b) which of the following best describes one mole of potassium sulfate?
a. it contains 6 atoms in total c. it contains 2 sulfur atoms
d. it contains 3.02 x 10^23 molecules
c) when two substances react to form products, the reactant that is used up first is called __
a. determining reactant c. limiting reactant
b. excess reactant d. catalytic reactant
q2. solve the following with proper steps.
a) a 9.20g sample of calcium oxide (cao) has 6.58g of calcium. the mass percent of oxygen in cao is __
b) the amount of moles of methane in 5.65 x 10^23 molecules of ch4 is __

Explanation:

Step1: Calculate mass of oxygen

Mass of oxygen \(m(O)=m(CaO)-m(Ca)\)
\(m(O)=9.20 - 6.58=2.62\space g\)

Step2: Calculate mass percent of oxygen

Mass percent formula \(\text{Mass}\%=\frac{m(\text{component})}{m(\text{total})}\times100\%\)
\(\text{Mass}\%(\text{O})=\frac{2.62}{9.20}\times 100\%\)
\(\text{Mass}\%(\text{O})=\frac{262}{9.20}\% \approx 28.5\%\)

Step3: Calculate moles of methane

Use Avogadro's number \(N = n\times N_A\), where \(N_A = 6.02\times10^{23}\space mol^{-1}\)
\(n=\frac{N}{N_A}\)
For \(N = 5.65\times 10^{23}\) molecules of \(CH_4\)
\(n=\frac{5.65\times 10^{23}}{6.02\times 10^{23}}\space mol\)
\(n\approx0.94\space mol\)

Answer:

a) The mass percent of oxygen in \(CaO\) is approximately \(28.5\%\)
b) The amount of moles of methane is approximately \(0.94\space mol\)