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Unformatted text preview: Solomon's Study Notes General Chemistry I
Fall 2011 Solomon Weiskop PhD Exercises: Mass/Mole, Percent Composition, Empirical Formula etc.
General Chemistry 1 Study Notes & Practice Problems are available to print out by registering at www.solomonlinetutor.com Solomon Weiskop PhD Copyright 2011 All Rights Reserved 1 The following is a selection of Exercises (with Detailed Solutions) covering some of the more challenging material discussed in the Study Notes on Mass & Mole. Working through these Exercises is a good way to strengthen your knowledge and problem-solving skills for this material. Try each of these Exercises on your own, only looking at its Detailed Solutions after you are convinced you have your best possible answer. (Many more problems on these topics are available in the Practice Problems.) [#1A] Determine the number of molecules in 325 mg of aspirin C9H8O4 [#1B] Determine the number of moles of hydrogen atoms in 325 mg of aspirin C9H8O4 Ans molecules aspirin B: moles H A: [#2] What is the mass percentage of oxygen in the following compound? Ans: 43.2% O [#3] A compound having a molar mass of 80 g/mol contains 60% oxygen by mass and the rest is sulfur. What is the molecular formula of this compound? Ans: 2 [# 4] The complete combustion of a hydrocarbon (MM= 88g/mol) produces 33g of CO2 and 18g H2O. (a)What is the empirical formula of this hydrocarbon? (b) What is the molecular formula of this hydrocarbon? Ans: (a) (b) [#5] The complete reaction of 16.12g of titanium with 23.88g of chlorine (Cl2) produces a compound with the formula TixCly. What is the empirical formula of the compound? Ans: [#6] Vitamin C contains only C H and O. Combustion of 1.000g of vitamin C produced 1.502 g CO2 and 0.409 g H2O. (a) What is the empirical formula of vitamin C? (b) The molar mass of vitamin C is 176 g/mol. What is its molecular formula? Ans: (a) (b) [#7] A certain metal M forms two oxides M 2O and MO. If the percent by mass of M in M2O is 70% what is the percent by mass of M in MO? Ans: 53.9% [#8] Mesitylene is a liquid hydrocarbon. If 0.1149g of this compound is burned in pure oxygen to give 0.3790g CO2 and 0.1035g H2O what is the empirical formula of the compound? Ans: 3 [#9] A hydrocarbon was found to consist of 10% hydrogen by mass. This result is compatible with which of the following compounds? Ans: E [#10] An element X forms an oxide X3O4 containing 90.67% X by mass. Calculate the approximate molar mass of the oxide. Ans: 686 g/mol 4 Detailed Solutions: [#1A] [#1B] [#2] The molecular formula of this compound is 5 [#3] Assuming 100 g of this compound, we have 60g O and 40g S. dividing by smallest number: The problem gives us 6 [#4] (a) Note that this compound is a hydrocarbon. That is, it contains only C and H (no O). divide by smallest number: To get integers, multiply by 3: (b) The problem gives us: 7 [#5] divide by smallest number [#6] (a) 8 divide by smallest number multiply by 3 to get integers (b) The problem gives us: [#7] 70% M (by mass) in M2O Plugging in MM(O) = 16g/mol and solving for MM(M): 9 [#8] Note that compound is a hydrocarbon. So it contains only C and H (no O). divide by smallest number: To get integers, multiply by 3: Note: The mass of the compound was provided in the problem but wasn't needed. If there had also been O in the compound, however, then you would need the mass of the compound (to solve for mass of O by subtraction). In any event, we can use the mass of the compound here as a check on our calculations: mass of compound = mass of C + mass of H 0.115 = 0.10336 + 0.0115 (small roundoff error) 10 [#9] We have 90% C and 10% H. So assuming 100g of compound we have 90g C and 10 g H dividing by smallest number: to get integers, multiply by 3 Now, looking at the line drawing provided: (A) MF = C5H8 EF = C5H8 (B) MF = C8H10 EF = C4H5 (C) MF = C6H10 EF = C3H5 (D) MF = C3H6 EF = CH2 (E) MF = C9H12 EF = C3H4 [#10] ...
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