Baking soda NaHCO3, is made from soda ash, a common name for sodium carbonate. The soda ash is obtained in two ways. It can be manufactured in a process in which carbon dioxide, ammonia, sodium chloride, and water are the starting materials. Alternatively, it is mined as a mineral called trona (top photo). Whether the soda ash is mined or manufactured, it is dissolved in water and carbon dioxide is bubbled through the solution. Sodium bicarbonate precipitates from the solution. As a chemical analyst you are presented with two samples of sodium bicarbonateâone from the manufacturing process and the other derived from trona. You are asked to determine which is purer and are told that the impurity is sodium carbonate. You decide to treat the samples with just sufficient hydrochloric acid to convert all the sodium carbonate and bicarbonate to sodium chloride, carbon dioxide, and water. You then precipitate silver chloride in the reaction of sodium chloride with silver nitrate. A 6.93 g sample of baking soda derived from trona gave 11.89 g of silver chloride. A6.78 g sample from manufactured sodium carbonate gave 11.77 g of silver chloride. Required:
Which sample is purer, that is, which has the greater mass percent NaHCO3?

Explanation:

From the given information:

mass of silver chloride AgCl = 11.89 g

molar mass of AgCl = 143.37 g/mol

We know that:

number of moles = mass/molar mass

∴

number of moles of AgCl = 11.89 g/ 143.37 g/mol

number of moles of AgCl = 0.0829 mol

The chemical equation for the mineral called trona is:

when being reacted with hydrochloric acid, we have:

One mole of NaCl formed from one mole of trona sample = 0.0829 moles of AgCl

i.e. 0.0829 moles of NaCl can be formed from AgCl

mass of trona sample = number of moles × molar mass

mass of trona sample = 0.0829 × 226

mass of trona sample = 18.735 g

The mass in the percentage of NaHCO₃ = mass of NaHCO₃/ mass of trona

The mass in the percentage of NaHCO₃ = 6.93/18.735

The mass in the percentage of NaHCO₃ = 0.36989

The mass in the percentage of NaHCO₃ = 36.99%

Nonetheless, a 6.78 g samples manufactured from sodium carbonate in pure 100%

∴

6.78 g sample manufactured from Na₂CO₃ is purer.

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