Question

silicon dioxide (sand) - find the mass of a scoop of sand using a weighing dish.
a) calculate the number of moles in the sample.
b) calculate the number of silicon dioxide molecules in the sample.
c) calculate the number of o atoms in the sample.

Explanation:

Step1: Calculate moles

Use formula \(n=\frac{m}{M}\), where \(m = 3.25g\), \(M=60.09g/mol\) (molar mass of \(SiO_2\)).
\(n=\frac{3.25}{60.09}\)

Step2: Calculate molecules

Use \(N = n\times N_A\), \(N_A=6.022\times10^{23}\) molecules/mol. First find \(n\approx0.0541\) mol (from step1). Then \(N = 0.0541\times6.022\times10^{23}\)

Step3: Calculate O atoms

Each \(SiO_2\) has 2 O atoms. So \(N_O=2\times N\)

Answer:

a) \(n=\frac{3.25}{60.09}\approx0.0541\) mol
b) \(N = 0.0541\times6.022\times10^{23}\approx3.26\times10^{22}\) molecules
c) \(N_O=2\times3.26\times10^{22}=6.52\times10^{22}\) O atoms