QuestionAn alkane X has a relative molecular mass of 86.0. Use C = 12.0 and H = 1.0.
(a) Find the molecular formula of X.
(b) Give the empirical formula of X.
(c) Give the molecular formula and Mr of the next member of the alkane homologous series.
(d) Give the molecular formula of the alkene with the same number of carbon atoms as X, and its Mr.Answers(a) Alkanes have the general formula CnH2n+2, so Mr = 12.0n + 1.0(2n + 2) = 14n + 2.
14n + 2 = 86.0
14n = 84, so n = 6. X is C6H14 (hexane, if unbranched). Check: 6 × 12.0 + 14 × 1.0 = 72.0 + 14.0 = 86.0.(b) 6 : 14 simplifies to 3 : 7, so the empirical formula is C3H7.(c) Successive members differ by CH2, so the next alkane is C7H16 with Mr = 86.0 + 14.0 = 100.0.(d) Alkenes have the general formula CnH2n. With n = 6 the alkene is C6H12, Mr = 72.0 + 12.0 = 84.0. It has two fewer hydrogen atoms than the alkane because of its C=C double bond.Tip: always write the general formula first and turn it into an expression for Mr. The same method works for any series, for example alcohols, where Mr = 14n + 18.
(a) Find the molecular formula of X.
(b) Give the empirical formula of X.
(c) Give the molecular formula and Mr of the next member of the alkane homologous series.
(d) Give the molecular formula of the alkene with the same number of carbon atoms as X, and its Mr.Answers(a) Alkanes have the general formula CnH2n+2, so Mr = 12.0n + 1.0(2n + 2) = 14n + 2.
14n = 84, so n = 6. X is C6H14 (hexane, if unbranched). Check: 6 × 12.0 + 14 × 1.0 = 72.0 + 14.0 = 86.0.(b) 6 : 14 simplifies to 3 : 7, so the empirical formula is C3H7.(c) Successive members differ by CH2, so the next alkane is C7H16 with Mr = 86.0 + 14.0 = 100.0.(d) Alkenes have the general formula CnH2n. With n = 6 the alkene is C6H12, Mr = 72.0 + 12.0 = 84.0. It has two fewer hydrogen atoms than the alkane because of its C=C double bond.Tip: always write the general formula first and turn it into an expression for Mr. The same method works for any series, for example alcohols, where Mr = 14n + 18.