White Powder in the Kitchen: Baking Soda or Washing Soda?
Two students are given a 10.0 g sample of a white kitchen powder that is known to contain sodium hydrogencarbonate and sodium chloride only. They place it in a dry test tube and heat it gently over a spirit lamp. A gas is evolved that turns limewater milky. Heating is continued until no further change occurs. The final residue has a mass of 7.4 g. Another group adds dilute hydrochloric acid directly to a fresh 2.0 g portion and collects the gas; they find that the gas also turns limewater milky. The teacher asks them to decide whether the original sample was pure baking soda, pure washing soda, or a mixture, and to calculate its composition. One student says the limewater test alone is enough; the other says the mass change is essential. The sample is not to be tasted, and the residue must be cooled in a desiccator before weighing.
1. Calculate the mass of sodium hydrogencarbonate and sodium chloride in the 10.0 g sample, and state whether it is pure baking soda. Show the equation and molar masses used.
Read the worked answer
Balanced equation: 2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g). Molar masses: NaHCO3 = 84 g/mol, Na2CO3 = 106 g/mol. For 2 mol NaHCO3 (168 g), the lost mass is 18 + 44 = 62 g. Observed loss = 10.0 − 7.4 = 2.6 g. Mass NaHCO3 = 2.6 × 168/62 = 7.05 g (about 7.0 g). Mass NaCl = 10.0 − 7.05 = 2.95 g. Check residue: 7.05 × 106/168 = 4.45 g Na2CO3; 4.45 + 2.95 = 7.40 g, which matches. Percentage NaHCO3 = 70.5%, NaCl = 29.5%. Pure baking soda would lose 10.0 × 62/168 = 3.69 g and leave 6.31 g, not 7.4 g, so the sample is not pure baking soda. Pure washing soda would not release CO2 on gentle heating, so the heating gas test also rules it out. A common error is to treat all 2.6 g as CO2; that gives about 9.9 g NaHCO3 and is inconsistent with the residue.
2. Explain why the hydrochloric acid test alone cannot distinguish baking soda from washing soda, and why the residue is cooled in a desiccator before weighing.
Read the worked answer
Both NaHCO3 and Na2CO3 react with dilute HCl to release CO2, which turns limewater milky; NaCl does not. Therefore the acid test shows that a carbonate or hydrogencarbonate is present, but it cannot decide which one is present or how much NaCl is mixed in. The heating test is more discriminating: NaHCO3 decomposes to give CO2 and water, whereas sodium carbonate does not release CO2 on gentle heating. The mass loss then gives a quantitative measure. The residue must be cooled in a desiccator because hot Na2CO3 can absorb moisture from the air, which would increase its mass and make the calculated bicarbonate content too low.
3. A student suggests passing the gas from heating through limewater and using the increase in mass of the limewater to find the bicarbonate content. Give one practical problem with this method and a better measurement.
Read the worked answer
Passing the gas through limewater and weighing the limewater increase is unreliable at school level: some CO2 escapes, some dissolves in water, water vapour from heating also passes through, and the limewater may not absorb all the gas. A better measurement is the mass loss of the reaction tube plus contents, or the mass of the solid residue after heating to constant mass, using a balance accurate to 0.01 g. Heating to constant mass means repeating the heat–cool–weigh cycle until two consecutive masses agree within the balance precision; this ensures decomposition is complete. The residue mass is then compared with the value predicted from the equation.