# Set the molecular formula of the substance, the vapor density of which for hydrogen = 13,

Set the molecular formula of the substance, the vapor density of which for hydrogen = 13, and the mass fractions of the elements are equal to C – 92.23%, H – 7.77%.

Given:
СxHy
D H2 (СxHy) = 13
ω (C in CxHy) = 92.23%
ω (H in СxHy) = 7.77%

Find:
СxHy -?

Solution:
1) Let m (CxHy) = 100 g;
2) m (C in CxHy) = ω (C in CxHy) * m (CxHy) / 100% = 92.23% * 100/100% = 92.23 g;
3) n (C in CxHy) = m (C in CxHy) / M (C) = 92.23 / 12 = 7.686 mol;
4) m (H in CxHy) = ω (H in CxHy) * m (CxHy) / 100% = 7.77% * 100/100% = 7.77 g;
5) n (H in CxHy) = m (H in CxHy) / M (H) = 7.77 / 1 = 7.77 mol;
6) x: y = n (C in CxHy): n (H in CxHy) = 7.686: 7.77 = 1: 1;
7) The simplest formula is CH;
8) M (CxHy) = D H2 (CxHy) * M (H2) = 13 * 2 = 26 g / mol;
9) M (CH) = Mr (CH) = Ar (C) * N (C) + Ar (H) * N (H) = 12 * 1 + 1 * 1 = 13 g / mol;
10) M (CxHy) / M (CH) = 2;
Substance – (CH) 2 – C2H2 – ethin.

Answer: Substance – C2H2 – ethin.

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