Question

pre - lab questions
answer questions 1 and 2 after you have read the experiment and before coming to class. show your work fully and clearly.

1. caffeine has the formula c₈h₁₀n₄o₂.

a. what is the empirical formula of caffeine?
b. calculate caffeines molar mass and its percent composition for c, h, n, and o.
c. calculate the mass of nitrogen contained in 0.845 g of caffeine.
d. calculate the mass of caffeine that contains 2.50 g of oxygen.

Explanation:

Step1: Find the empirical formula

Divide each sub - script by the greatest common divisor. For $C_8H_{10}N_4O_2$, the greatest common divisor is 2. So the empirical formula is $C_4H_5N_2O$.

Step2: Calculate molar mass

The molar mass of $C$ is $12.01\ g/mol$, $H$ is $1.01\ g/mol$, $N$ is $14.01\ g/mol$, and $O$ is $16.00\ g/mol$.
For $C_8H_{10}N_4O_2$:
\[M=(8\times12.01 + 10\times1.01+4\times14.01 + 2\times16.00)\ g/mol\]
\[M=(96.08+10.1 + 56.04+32.00)\ g/mol=194.22\ g/mol\]

Step3: Calculate percent composition

Percent of $C$: \(\frac{8\times12.01}{194.22}\times100\%=\frac{96.08}{194.22}\times100\%\approx49.47\%\)
Percent of $H$: \(\frac{10\times1.01}{194.22}\times100\%=\frac{10.1}{194.22}\times100\%\approx5.20\%\)
Percent of $N$: \(\frac{4\times14.01}{194.22}\times100\%=\frac{56.04}{194.22}\times100\%\approx28.86\%\)
Percent of $O$: \(\frac{2\times16.00}{194.22}\times100\%=\frac{32.00}{194.22}\times100\%\approx16.48\%\)

Step4: Calculate mass of nitrogen in 0.845 g of caffeine

The mass fraction of $N$ in caffeine is \(\frac{4\times14.01}{194.22}\).
The mass of $N$ in 0.845 g of caffeine is \(m_N = 0.845\ g\times\frac{4\times14.01}{194.22}\)
\[m_N=0.845\ g\times\frac{56.04}{194.22}\approx0.243\ g\]

Step5: Calculate mass of caffeine with 2.50 g of oxygen

The mass fraction of $O$ in caffeine is \(\frac{2\times16.00}{194.22}\).
Let the mass of caffeine be $m$. Then \(\frac{2\times16.00}{194.22}\times m = 2.50\ g\)
\[m=\frac{2.50\ g\times194.22}{2\times16.00}\approx15.17\ g\]

Answer:

a. $C_4H_5N_2O$
b. Molar mass: $194.22\ g/mol$, Percent composition: C: $\approx49.47\%$, H: $\approx5.20\%$, N: $\approx28.86\%$, O: $\approx16.48\%$
c. $\approx0.243\ g$
d. $\approx15.17\ g$