Fundamentals Of Chemistry: Subchapter, Stoichiometry And Units

Fundamentals Of Chemistry: Subchapter, Stoichiometry And Units

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Ch. 1 Fundamentals of Chemistry
Subchapter – Stoichiometry and Units
1. An element X forms an oxide with the formula X4O10. If 31 g of X combines with 40 g of oxygen, what is the identity of X?
A. N
B. P
C. C
D. S
Correct Answer: B: P
Explanation:
We can solve for the atomic mass of X to identify it. The amount in g of X for any amount in g of O is given by:
Rearranging the above equation and substituting the given mass of X and O, we get:
Thus, the atomic mass of X is 31 g/mol, which corresponds to phosphorus.
Subchapter – Stoichiometry and Units
2. Nitrogen gas reacts with oxygen gas to form nitrogen dioxide. How many grams of nitrogen gas are needed to react completely with 8.0 g of oxygen gas?
A. 7.0 g…show more content… D: 51.8 g
If we used 5/1 instead of 1/5 as the molar ratio, then we will get this answer.
Subchapter – Moles, Molecular Weight
23. A sample of soda ash contains 50% Na2CO3 (106 g/mol) by weight and the other 50% are inert substances. What mass of CO2 (44 g/mol) is formed when 11 g of the soda ash reacts with excess HCl according to the reaction below?
Na2CO3 + 2 HCl → 2 NaCl + H2O + CO2
A. 4.6 g
B. 242 g
C. 2.3 g
D. 5.5 g
Correct Answer – C: 2.3 g
Explanation:
C: 2.3 g
To calculate for the amount of CO2 formed:
11/106 is approximately 0.1. Thus, the answer should be close to 2.2 g.
A: 4.6 g
If we did not account for the purity of the sample, then we will arrive at this answer.
B: 242 g
This answer would have been chosen if the mass of the sample was not converted into moles.
D: 5.5 g
This is the mass of Na2CO3 in the sample, and not of CO2.
Subchapter – Moles, Molecular Weight
24. What mass of aluminum (27 g/mol) is needed to completely react with 14.2 g of Cl2 (71 g/mol) according to the reaction Al + Cl2 → AlCl3?
A. 5.4 g
B. 8.1 g
C. 2.7 g
D. 3.6 g
Correct Answer – D: 3.6 g
Explanation:
D: 3.6