Question

question 2
the formula of nitrobenzene is c6h5no2. the molecular weight of this compound is ______ g/mol.
109.10
43.03
123.11
107.11
3.06
question 3
the mass % of c in methane (ch4) is ______.
7.743
133.6
25.13
92.26
74.87

Explanation:

Question 2

Step1: Calculate the molecular weight of each element in nitrobenzene ($C_6H_5NO_2$)

• Carbon ($C$): There are 6 carbon atoms. The atomic weight of $C$ is approximately $12.01\ g/mol$. So the total weight from carbon is $6\times12.01 = 72.06\ g/mol$.
• Hydrogen ($H$): There are 5 hydrogen atoms. The atomic weight of $H$ is approximately $1.008\ g/mol$. So the total weight from hydrogen is $5\times1.008=5.04\ g/mol$.
• Nitrogen ($N$): There is 1 nitrogen atom. The atomic weight of $N$ is approximately $14.01\ g/mol$.
• Oxygen ($O$): There are 2 oxygen atoms. The atomic weight of $O$ is approximately $16.00\ g/mol$. So the total weight from oxygen is $2\times16.00 = 32.00\ g/mol$.

Step2: Sum up the weights of all elements

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Question 3

Step1: Calculate the molecular weight of methane ($CH_4$)

• Carbon ($C$): Atomic weight is $12.01\ g/mol$.
• Hydrogen ($H$): There are 4 hydrogen atoms. The atomic weight of $H$ is $1.008\ g/mol$. So the total weight from hydrogen is $4\times1.008 = 4.032\ g/mol$.

The molecular weight of $CH_4$ is $12.01+4.032=16.042\ g/mol$.

Step2: Calculate the mass percentage of carbon

The formula for mass percentage is $\text{Mass}\%=\frac{\text{Mass of element}}{\text{Mass of compound}}\times100\%$.
For carbon in $CH_4$, $\text{Mass}\%=\frac{12.01}{16.042}\times 100\% \approx 74.87\%$

Answer:

Question 2: 123.11 g/mol (the third option).
Question 3: 74.87 (the fifth option).