Aldehydes and ketones — practice questions

Method: Add about 1 cm³ of Tollens' reagent to a clean test tube containing a few drops of each unknown liquid. Warm the mixture gently in a water bath for a few minutes. [1 mark]

Observations:

• With pentanal (the aldehyde), a silver mirror will form on the inside of the test tube. [1 mark]
• With pentan-2-one (the ketone), there will be no visible change / the solution will remain colourless. [1 mark]

Step 1: Identify the product and write the equation. Propanal is an aldehyde, so it will be reduced to a primary alcohol, propan-1-ol. CH₃CH₂CHO + 2[H] $\rightarrow$ CH₃CH₂CH₂OH [1 mark]

Step 2: Calculate the molar mass of propanal. $M_r$(propanal) = (3 × 12.0) + (6 × 1.0) + 16.0 = 58.0 g mol⁻¹ [1 mark]

Step 3: Calculate the moles of propanal. Moles = mass / $M_r$ = 2.90 g / 58.0 g mol⁻¹ = 0.0500 mol [1 mark]

Step 4: Use the stoichiometry to find the moles of the product. The mole ratio of propanal to propan-1-ol is 1:1. Moles of propan-1-ol = 0.0500 mol.

Step 5: Calculate the mass of the product. $M_r$(propan-1-ol) = (3 × 12.0) + (8 × 1.0) + 16.0 = 60.0 g mol⁻¹ Mass = moles × $M_r$ = 0.0500 mol × 60.0 g mol⁻¹ = 3.00 g [1 mark]

Maximum mass of propan-1-ol = 3.00 g.