The difference in molecular mass between an alkene and alkyne with six carbon per mole is

1. Analyze the Question

We need to find the difference in the molecular mass between an alkene and an alkyne, both containing six carbon atoms per molecule.

2. Explain the Concept

• General Formula for Alkenes: $C_nH_{2n}$
• General Formula for Alkynes: $C_nH_{2n-2}$
• Atomic Masses: Carbon (C) = 12, Hydrogen (H) = 1.
• The difference in mass arises from the difference in the number of hydrogen atoms, as the number of carbon atoms is the same for both.

3. Show the Solution

Step 1: Determine the molecular formulas for $n = 6$

• Alkene ($C_6H_{2n}$): $C_6H_{2(6)} = C_6H_{12}$
• Alkyne ($C_6H_{2n-2}$): $C_6H_{2(6)-2} = C_6H_{10}$

Step 2: Calculate the molecular masses

• Mass of Alkene ($C_6H_{12}$): $(6 \times 12) + (12 \times 1) = 72 + 12 = 84$
• Mass of Alkyne ($C_6H_{10}$): $(6 \times 12) + (10 \times 1) = 72 + 10 = 82$

Step 3: Find the difference

• Difference = $84 - 82 = 2$

(Note: Alternatively, you can simply look at the difference in the number of hydrogen atoms. An alkene has 2 more hydrogen atoms than an alkyne. Since each hydrogen atom has a mass of 1, the difference is $2 \times 1 = 2$.)

4. Identify the Answer

Comparing our result to the options provided, the correct choice is C.

5. Exam Tip

Whenever you are asked for the difference in molecular mass between members of different homologous series with the same number of carbon atoms, don't waste time calculating the full molecular mass of the carbon atoms. Simply find the difference in the number of hydrogen atoms and multiply by the mass of hydrogen (1). It saves time and reduces the chance of arithmetic errors!