1.A student attempts to identify an unknown compound by using chemical reactions
ID: 534400 • Letter: 1
Question
1.A student attempts to identify an unknown compound by using chemical reactions to calculate mass relationships. After heating a sample weighing 0.4521 g, the mass decreased to 0.3400 g. When the product was reacted to form a chloride, the resulting mass was 0.3304 g.
How many moles of the chloride salt would be produced from one mole of the original unknown compound?
2. A student attempts to identify an unknown compound by using chemical reactions to calculate mass relationships. After heating a sample weighing 0.4521 g, the mass decreased to 0.3400 g. When the product was reacted to form a chloride, the resulting mass was 0.3304 g.
Given the mole ratio from the previous question, how many grams of sodium chloride salt can be produced from one mole of the unknown sodium compound? (One mole of the substance is equivalent to the formula weight of the substance.
__________g unknown Na compound (1 mol) = ________g of sodium chloride
3. A student attempts to identify an unknown compound by using chemical reactions to calculate mass relationships. After heating a sample weighing 0.4521 g, the mass decreased to 0.3400 g. When the product was reacted to form a chloride, the resulting mass was 0.3304 g.
Given the mole ratio from the previous question, how many grams of potassium chloride salt can be produced from one mole of the unknown potassium compound? (One mole of the substance is equivalent to the formula weight of the substance.
_____g unknown K compound (1 mol) = _______g of potassium chloride
4. What is the theoretical mass ratio to the correct number of significant figures?
mass of chloride.mass of unknown compound
Answer for the Na:
5. What is the theoretical mass ratio to the correct number of significant figures?
mass of chloride.mass of unknown compound
Answer for the K:
6. What is the unknown sample mass ratio?
(mass of chloride/mass of unknown compound)
Compare this answer with the answers to the previous 2 questions
Answer:
What is the unknown in this example? sodium chloride, sodium carbonate, potassium chloride, potassium carbonate, sodium bicarbonate
Explanation / Answer
We must note one point given in the question. The original compound gives either sodium chloride (NaCl) or potassium chloride (KCl) on treatment with a chloride salt. Therefore, the compound must be a Na or K containing compound.
The original compound loses in mass on heating. It is given that the original compound can only be a carbonate or a bicarbonate salt. Since carbonates are stable and do not decompose on heating, hence the compound is a bicarbonate. Since we can have only Na or K as the metal, the compound is either sodium bicarbonate (NaHCO3) or potassium bicarbonate (KHCO3).
The balanced chemical equations for the decomposition of NaHCO3 or KHCO3 are as below:
2 NaHCO3 (s) --------> Na2CO3 (s) + CO2 (g) + H2O (g)
2 KHCO3 (s) -------> K2CO3 (s) + CO2 (g) + H2O (g)
1) Let us denote a chloride salt as MCl2 where M is a divalent metal. We can write down the balanced chemical equations for the reactions of NaHCO3 or KHCO3 with MCl2 as below:
2 NaHCO3 + MCl2 -------> 2 NaCl + M(HCO3)2
2 KHCO3 + MCl2 -------> 2 KCl + M(HCO3)2
As per the stoichiometric reactions above,
2 moles NaHCO3 = 2 moles NaCl
2 moles KHCO3 = 2 moles KCl.
Therefore, 1 mole NaHCO3 or 1 mole KHCO3 = 1 mole NaCl or 1 mole KCl.
Therefore, 1 mole of chloride salt can be produced from 1 mole of the original bicarbonate salt.
2) We need to find out the molar masses of NaHCO3 and NaCl.
Molar mass of NaHCO3 = (22.989 + 1.008 + 12.01 + 3*15.9994) g/mol = 84.0052 g/mol.
Molar mass of NaCl = (22.989 + 35.453) g/mol = 58.442 g/mol.
Therefore, 84.0052 g Na compound (1 mole) = 58.442 g sodium chloride (ans).
3) We need to find out the molar masses of KHCO3 and KCl.
Molar mass of KHCO3 = (39.0983 + 1.008 + 12.01 + 3*15.9994) g/mol = 100.1145 g/mol.
Molar mass of KCl = (39.0983 + 35.453) g/mol = 74.5513 g/mol.
Therefore, 100.1145 g K compound (1 mole) = 74.5513 g potassium chloride (ans).
4) The theoretical mass ratio is given as
Mass Ratio = (mass of 1 mole of NaCl)/(mass of 1 mole of NaHCO3) = (58.442 g)/(84.0052 g) = 0.695 0.69 (ans).
5) The theoretical mass ratio is given as
Mass Ratio = (mass of 1 mole of KCl)/(mass of 1 mole of KHCO3) = (74.5513 g)/(100.1145 g) = 0.744 0.74 (ans).
6) Ratio of the masses in (4) and (5) = 0.69:0.74 = 1:1.07 1:1 (ans).
The compound is most likely NaHCO3 or KHCO3.
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