QuestionA hydrocarbon Y contains 85.7% carbon by mass. Its relative molecular mass is 56.0. Use C = 12.0 and H = 1.0.
(a) Find the empirical formula of Y.
(b) Find the molecular formula of Y.
(c) Y is an open-chain compound. State whether it is saturated and suggest a structural formula and name.Answers(a) Y is a hydrocarbon, so the rest of the mass is hydrogen: 100 - 85.7 = 14.3%. Divide each percentage by the relative atomic mass:
C: 85.7 / 12.0 = 7.14 H: 14.3 / 1.0 = 14.3
Divide by the smaller number: C = 7.14 / 7.14 = 1, H = 14.3 / 7.14 = 2.00. The empirical formula is CH2.(b) The empirical formula mass is 12.0 + 2 × 1.0 = 14.0. The factor is 56.0 / 14.0 = 4, so the molecular formula is 4 × CH2 = C4H8.(c) C4H8 fits CnH2n. The alkane with four carbons is C4H10, so Y has two fewer hydrogen atoms. As it is an open chain, Y must contain a C=C double bond, so it is unsaturated (an alkene). One possible structure is CH2=CHCH2CH3, but-1-ene. CH3CH=CHCH3 (but-2-ene) is also a correct answer.Tip: keep at least three significant figures in the division step. Rounding 7.14 or 14.3 too early can give a wrong ratio.
(a) Find the empirical formula of Y.
(b) Find the molecular formula of Y.
(c) Y is an open-chain compound. State whether it is saturated and suggest a structural formula and name.Answers(a) Y is a hydrocarbon, so the rest of the mass is hydrogen: 100 - 85.7 = 14.3%. Divide each percentage by the relative atomic mass:
Divide by the smaller number: C = 7.14 / 7.14 = 1, H = 14.3 / 7.14 = 2.00. The empirical formula is CH2.(b) The empirical formula mass is 12.0 + 2 × 1.0 = 14.0. The factor is 56.0 / 14.0 = 4, so the molecular formula is 4 × CH2 = C4H8.(c) C4H8 fits CnH2n. The alkane with four carbons is C4H10, so Y has two fewer hydrogen atoms. As it is an open chain, Y must contain a C=C double bond, so it is unsaturated (an alkene). One possible structure is CH2=CHCH2CH3, but-1-ene. CH3CH=CHCH3 (but-2-ene) is also a correct answer.Tip: keep at least three significant figures in the division step. Rounding 7.14 or 14.3 too early can give a wrong ratio.