chemistry (pls show all ur step obtaining)?

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.

Alkane is 82.76% C and 17.24%H
Take 100g compound
Mol C = 82.76 / 12.011= 6.89 mol C
Mol H in 17.24g = 17.24 / 1.008 = 17.103

Divide through by smaller:
C = 0.403 mol C
H = 1.00 mol H

Resolve fractions , multiply through by 10
C = 4.03
H = 10.0

Alkane is C4H10

CH3CH2CH2CH3