# 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