Question

1. A student attempts to identify an unknown compound by the method used in this experiment. She finds that when she heated a sample weighing 0.5015 g the mass went down appreciably, to 0.3432 g. When the product was converted to a chloride, the mass went up, to 0.3726 g.

a. Is the sample a carbonate? ____________ Give your reasoning.

b. What are the two compounds that might be in the unknown? ____________ and ____________

c. Write the chemical equation for the overall reaction that would occur when the original compound was converted to a chloride. If the compound is a hydrogen carbonate, use the sum of Reactions 1 and 2. If the sample is a carbonate, use Reaction 2.

Write the equation for a sodium salt and then for a potassium salt.

d. How many moles of the chloride salt would be produced from one mole of original compound? ____________

e. How many grams of the chloride salt would be produced from one molar mass of original compound? Molar masses: NaHCO3 ____________ g Na2CO3 ____________ g NaCl ____________ g KHCO3 ____________ g K2CO3 ____________ g KCl ____________ g If a sodium salt, ____________ g original compound → ____________ g chloride If a potassium salt, ____________ g original compound → ____________ g chloride

f. What is the theoretical value of Q, if she has an Na salt? ____________ if she has a K salt? ____________ g.

What was the observed value of Q? ____________

h. Which compound did she have as an unknown?

Answer 1

a) Is the sample a carbonate?

Answer: No

b) What are the two compounds that might be the unknown?

Answer: The unknowns might be NaHCO₃ and KHCO₃

c) Write the chemical equation for the overall reaction that would occur when the original compound was converted to a chloride. If the compound is a hydrogen carbonate, use the sum of Equations 1 and 2. If the sample is a carbonate, use Equation 2. Write the equation for a sodium salt and then for a potassium salt?

2NaHCO₃    Na₂CO₃ + CO₂ + H₂O

2KHCO₃   K₂CO₃ + CO₂ +H₂O

Na₂CO₃(s) + 2H⁺(aq) + 2Cl(aq)   2NaCl(s) + H₂O(g) +CO₂(g)

K₂CO₃(s) + 2H⁺(aq) + 2Cl(aq) 2KCl(s) + H₂O(g) + CO₂(g)

d) How many moles of the chloride salt would be produced from one mole of the original compound?

Answer: 1 mole

e) How many grams of the chloride salt would be produced from one molar mass of the original compound?

We have, Molar mass of NaHCO₃ = 84.007g

Molar mass of Na₂CO₃ = 106g

Molar mass of NaCl = 58.44 g

Molar mass of KHCO₃ = 100.16g

Molar mass of K₂CO₃ = 138.21g

Molar mass of KCl = 74.55 g

If a sodium salt = 84.007g, original compound = 54.44g chloride

If a potassium salt =100.16g original compound = 74.55g chloride

f) What is the theoretical mass ratio, mass chloride/mass original compound?

If she has a Na salt = 0.30

If she has a K salt = 0.26

g) What was the Value of Q?

Answer: 0.257

h) Which compound did she have as an unknown?

Answer: KHCO₃