✔ 最佳答案
Your answer is incorrect.
This is because you did not consider the hydrogen in the alkane molecule.
You have assume that there were 100 g of the alkane and you have got n = 7
If you assumed that there were 1000 g of the alkane using the same method, you would got n = 70
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Method 1 :
Assume that there are 100 g CₙH₂ₙ₊₂.
Mass of C = (100 g) × 82.76% = 82.76 g
Mass of H = (100 g) × (100 - 82.76)% = 17.24 g
No. of moles of C = (82.76 g) / (12 g/mol) = 6.897 mol
No. of moles of H = (17.24 g) / (1.0 g/mol) = 17.24 mol
Mole ratio C : H
n : (2n + 2) = 6.897 : 17.24
17.24n = 13.79n + 13.79
3.45n = 13.79
n = 4.00
n = 4
2n + 2 = 10
The molecular formula of the alkane is C₄H₁₀.
The condensed structural formula of alkane is CH₃CH₂CH₂CH₃.
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Method 2:
Mass of 1 mole of the alkane = [12.0×n + 1.0×(2n+2)] g = 14n + 2
Mass of C in 1 mole of the alkane = 12.0×n = 12n
Percent by mass of C in the alkane :
[12n/(14n + 2)] × 100% = 82.76%
12n/(14n + 2) = 82.76/100
1200n = 82.76(14n + 2)
1200n = 1158.64n + 165.52
41.36n = 165.52
n = 4.00
n = 4
2n + 2 = 10
The molecular formula of the alkane is C₄H₁₀.
The condensed structural formula of alkane is CH₃CH₂CH₂CH₃.
