Types of formula for organic molecules

4 marks · Normal

But-2-yne is an organic molecule containing four carbons and a triple bond.

(a)

Give the molecular formula for but-2-yne.

[1 mark]

(b)

Give the empirical formula for but-2-yne.

[1 mark]

(c)

Draw the displayed formula for but-2-yne.

[2 marks]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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(a) C4H6

(b) C2H3

(c) Displayed formula: CH3–C≡C–CH3 (shown with four carbons, triple bond between central two carbons, all correct hydrogens)

Must show: C4H6, C2H3, Displayed formula with four carbons, triple bond between C2 and C3

Common mistakes: Triple bond in wrong position; Wrong number of hydrogens

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · Molecular formula is correct: C4H6.

B2 (b) · 1 mark · Empirical formula is correct: C2H3.

M1 (c) · 1 mark · Structure shows four carbons connected with a triple bond in the correct position (between C2 and C3).

A1 (c) · 1 mark · All hydrogen atoms are correctly placed.

Worked solution

a) The molecular formula for but-2-yne is C4H6.

b) The empirical formula is C2H3.

c) Displayed formula: