JAMB Chemistry 2012 Paper

Concept:Carbohydrates can be written as $\mathrm{C}_x(\mathrm{H}_2\mathrm{O})_y$.We compare the values of $x$ and $y$ for each compound in the pairs.Explanation:Glucose and fructose both have the formula $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 = \mathrm{C}_6(\mathrm{H}_2\mathrm{O})_6$.So for glucose and fructose, $x = 6$ and $y = 6$, meaning $x = y$.Maltose and sucrose both have the formula $\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11} = \mathrm{C}_{12}(\mathrm{H}_2\mathrm{O})_{11}$.So for maltose and sucrose, $x = 12$ and $y = 11$, meaning $x \ne y$.Galactose has the same formula as glucose: $x = y$.Starch has the repeating unit $\mathrm{C}_6\mathrm{H}_{10}\mathrm{O}_5 = \mathrm{C}_6(\mathrm{H}_2\mathrm{O})_5$.So for starch, $x = 6$ and $y = 5$, meaning $x \ne y$.The only pair where both compounds have $x \ne y$ is maltose and starch.Answer:B. maltose and starch