IChO 2003 · Theory problem 3

Statements about a glucose solution

The molar mass of glucose (\(\ce{C6H12O6}\)) is 180 g mol\(^{-1}\) and \(N_A\) is the Avogadro constant. Which one of the following statements is not correct?

(a) An aqueous 0.5 M solution of glucose is prepared by dissolving 90 g of glucose to give 1000 cm\(^3\) of solution. ( )

(b) 1.00 mmol amount of glucose has a mass of 180 mg. ( )

(c) 0.0100 mol of glucose comprises of \(0.0100\times24\times N_A\) atoms. ( )

(d) 90.0 g glucose contain \(3\times N_A\) atoms of carbon. ( )

(e) 100 cm\(^3\) of a 0.10 M solution contain 18 g of glucose. ( )

Official English text of the problem.

Answers

  • (a) Correct.
  • (b) Correct.
  • (c) Correct.
  • (d) Correct.
  • (e) Incorrect: the solution contains 1.8 g of glucose.

Solution

Glucose, \(\ce{C6H12O6}\), has the open-chain connectivity SMILES O=CC(O)C(O)C(O)C(O)CO. Stereochemistry is omitted because it does not affect these calculations.

We use \[ n=\frac{m}{M},\qquad c=\frac{n}{V}, \] where \(M=180\ \mathrm{g\,mol^{-1}}\) and \(V\) is the final solution volume in litres.

(a) The amount of glucose is \[ n=\frac{90\ \mathrm{g}}{180\ \mathrm{g\,mol^{-1}}} =0.5\ \mathrm{mol}. \] Since \(1000\ \mathrm{cm^3}=1\ \mathrm{L}\), \[ c=\frac{0.5\ \mathrm{mol}}{1\ \mathrm{L}} =0.5\ \mathrm{mol\,L^{-1}}=0.5\ \mathrm{M}. \] The statement is correct.

(b) Converting millimoles to moles gives \[ m=nM =(1.00\times10^{-3}\ \mathrm{mol}) (180\ \mathrm{g\,mol^{-1}}) =0.180\ \mathrm{g}=180\ \mathrm{mg}. \] The statement is correct.

(c) Each glucose molecule contains \[ 6+12+6=24\ \text{atoms}. \] The sample contains \(0.0100N_A\) glucose molecules, so its total number of atoms is \[ N_{\text{atoms}}=24\times0.0100N_A=0.240N_A. \] The statement is correct.

(d) The amount of glucose is \[ n=\frac{90.0\ \mathrm{g}}{180\ \mathrm{g\,mol^{-1}}} =0.500\ \mathrm{mol}. \] Each glucose molecule contains six carbon atoms. Therefore, the sample contains \[ N_{\text{carbon atoms}} =6\times0.500N_A=3.00N_A. \] The statement is correct.

(e) The solution volume is \[ 100\ \mathrm{cm^3}=0.100\ \mathrm{L}. \] Hence, \[ n=cV =(0.10\ \mathrm{mol\,L^{-1}})(0.100\ \mathrm{L}) =0.010\ \mathrm{mol}, \] and \[ m=nM =(0.010\ \mathrm{mol})(180\ \mathrm{g\,mol^{-1}}) =\boxed{1.8\ \mathrm{g}}. \] The stated mass of 18 g is ten times too large. Thus, (e) is the single incorrect statement.

The answers were checked against the official answers published by the organisers.