C. Incorrect! This number is too large for moles. Divide the given number with Avogadro s

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1 AP Chemistry - Problem Drill 09: The Mole No. 1 of Avogadro s number is defined as a number equal to the number of atoms in 12 grams of the carbon-12. This number is 6.02x This is one of the most important numbers in chemistry. It is used to count atoms or molecules in any substance. How many moles are equal to molecules? (A) mole (B) 473 mole (C) mole (D) mole You divided the numbers the wrong way. Check your set-up and recalculate the division. It is a one-step process. B. Correct! Good job! Take the given number of molecules and divide by the Avogadro s number. The mole number is usually within a few orders of magnitude of 1. You converted molecules to moles correctly. This number is too large for moles. Divide the given number with Avogadro s number needs. Use the exponent key (EE or EXP) when calculating with scientific notation. D. Incorrect! This number is too large for moles. Divide the given number with Avogadro s number needs. Use the exponent key (EE or EXP) when calculating with scientific notation. 1 mole = molecules. Set up the dimensional analysis and cancel the unit. Run the calculation molecules 1 mole molecules = _473 mole There are total of 473 moles in 2.85 x molecules, whatever the molecule is. The correct answer is (B).

2 No. 2 of The molar mass is defined as the mass of an element or compound divided by its moles, typically in the unit of g/mol. The molar mass of a compound is equal to the sum of the atomic masses of the atoms in a molecule. Find the molar mass of Fe 2 (SO 4 ) 3. (A) g/mol (B) g/mol (C) g/mol (D) g/mol You multiply the subscripts outside and inside the parenthesis, not add them. There are two iron atoms, not just one. The sum is miscalculated. Add the atomic masses of all atoms together. Multiply the subscript outside the parenthesis by the subscripts inside the parenthesis to get the total atom count. You need to multiply the subscript outside the parenthesis by the subscripts inside the parenthesis to get the total atom count. Good job! You correctly calculated the molar mass of the compound. Add the atomic masses of all atoms together. Multiply the subscript outside the parenthesis by the subscripts inside the parenthesis to get the total atom count. Molar mass is the sum of all atomic masses. Add the atomic masses of all atoms together. Multiply the subscript outside the parenthesis by the subscripts inside the parenthesis to get the total atom count. Fe = S = O = g/mol

3 No. 3 of The most common conversion in chemistry is mass-to-mole. The mole is defined as the ratio of the mass (g) and molar mass (g/mol). How many molecules are in g of H 2 O? (A) molecules (B) molecules (C) molecules (D) molecules You converted grams to moles, but you need to take it one step farther to get to molecules. This is a two-step conversion. Check your calculations again. Convert the mass to mole and apply the Avogadro s number to calculate the number of molecules. C. Correct! Good job! You successfully converted grams to molecules. It is a two-step process. Convert the mass to mole and apply the Avogadro s number to calculate the number of molecules. D. Incorrect! There are two hydrogen atoms in a water molecule. Convert the mass to mole and apply the Avogadro s number to calculate the number of molecules. This takes two steps. Convert the mass to mole and apply the Avogadro s number to calculate the number of molecules. Molar mass = grams for 1 mole 1 mole = molecules Hydrogen 2 molecules 1.01 g/mole H = 2.02 g/mole of H 2 Oxygen 1 molecule g/mole = g/mole g/mole g H 2 O 1 mole H 2 O g H 2 O molecules H 2 O 1 mole H 2 O = molecules H 2 O The correct answer is (C).

4 No. 4 of The percent composition of a component in a compound is the percent of the total mass of the compound that is due to that component. Find the percent composition, by mass, of copper in Cu(CH 3 COO) 2? (A) % (B) % (C) % (D) % Check the calculation carefully. To find percent by mass, divide the mass of copper by the mass of the entire compound. Check the calculation carefully. To find percent by mass, divide the mass of copper by the mass of the entire compound. Check the calculation carefully. You need to distribute the subscript outside the parenthesis to the atoms inside the parenthesis when finding the whole molar mass. You correctly calculated the percent copper by mass. To find percent by mass, divide the mass of copper by the mass of the entire compound. % = (part whole) 100% The part is the mass of Cu in the compound. The whole is the molar mass of the whole compound Cu = C = H = 6.06 O = g/mole % Cu = ( ) 100% = 34.98%

5 No. 5 of The empirical formula is the simplest formula of a compound. The molecular formula is the same as or a multiple of the empirical formula. Find the empirical formula and molecular formula if the sample is composed only of nitrogen and oxygen. There is 30.4% of nitrogen. The sample molecule has a molar mass of 92 g/mole? (A) NO 2, N 2 O 4 (B) NO 2, NO 2 (C) NO 2, N 3 O 6 (D) N 2 O 4, NO 2 A. Correct! Good job! Calculate the moles of each element and take the simplest ratio for the empirical formula. Divide the molar mass of the two formulas to get the multiple. You correctly determined the empirical formula, but the molecular formula's mass is double that, not the same. You correctly determined the empirical formula, but the molecular formula's mass is double that, not triple. D. Incorrect! You have the empirical and molecular formulas flipped. Empirical formula is the lowest whole number ratio of the moles of each atom If percent composition is given, use the percents as grams. The molecular formula is the actual ratio of atoms found by finding the ratio of the empirical formula s molar mass to the molecular formula s molar mass g N 1 mole N g N 69.6 g O 1 mole g O = 2.17 mole N = 4.35 mole O 2.17 mole N 2.17 mole 4.35 mole N 2.17 mole = 1 N = 2 O Empirical formula = NO 2 Empirical formula s molar mass: N = O = g/mole 92 g/mole = 2 46 g/mole Molecular formula = NO 2 2 = N 2 O 4 The correct answer is (A).

6 No. 6 of Oxygen constitutes most of the mass of living organisms, because it is their major constituent. Oxygen, by mass, is the third most abundant element in the universe, after hydrogen and helium. How many moles of oxygen atoms are in 4 moles of O 2? (A) 2 (B) 4 (C) 6 (D) 8 There are 2 moles of oxygen atoms in each mole of O 2, but this problem is asking about 4 moles of O 2. There are 2 moles of oxygen atoms in each mole of O 2. There are 2 moles of oxygen atoms in each mole of O 2. Good job! There are 2 moles of oxygen atoms in each mole of O 2 and this problem asked about 4 moles of O 2 (2*4 = 8). There are 2 moles of oxygen atoms in each mole of O 2 and this problem asked about 4 moles of O 2, which would have 8 oxygen atoms.

7 No. 7 of Dinitrogen pentaoxide is a white crystalline oxide of nitrogen and regarded as the anhydride (removal of water from an acid) of nitric acid, 2HNO 3 N 2 O 5 + H 2 O. Calculate the number of moles of oxygen atoms in 1.20 x10 25 N 2 O 5 molecules? (A) 19.9 mol (B) 39.8 mol (C) 6.0 x mol (D) 99.5 mol This is the number of moles for the molecule, not O atom. It is a two-step calculation. Convert the molecular moles to atomic moles. Almost there! You multiplied the molecular moles by 2. This in fact is the number of moles for N atoms, not O atoms. Read the question carefully. This is the number of N atoms, not the moles. Check the reasonableness of the answer from the order of magnitude. Good job! This is a two-step process. Convert the number of molecules into the number of moles for the molecules. Multiply by 5 to get the number of O atoms. This is a two-step process. First, you need to calculate the mole number of the molecule N 2 O 5 using the Avogadro s number. #Mole (N 2 O 5 ) = 1.20x10 25 /6.02x10 23 = 19.9 moles Secondly, using the molecular formula given, you can relate the moles of the molecule N 2 O 5 to the moles of the atom O. There are 5 O atoms in every molecule, i.e. for one mole of N 2 O 5, there is 5 moles of O atoms. #Mole O = 5 x #Mole (N 2 O 5 ) = 5 x 19.9 = 99.5 moles Note that the number of significant figures (3) of the result is the same in the original data.

8 No. 8 of An empirical formula is a chemical formula that represents the relative proportions of the elements in a molecule, rather than the actual number of atoms of the elements. Which of the following is a possible molecular formula with the empirical formula CH 2 O? (A) C 2 H 5 OH (B) CH 3 CHO (C) C 6 H 5 COOH (D) C 6 H 12 O 6 The empirical formula s atomic ratio is 1:2:1 for C:O:H. However, the ethanol C 2 H 5 OH has 2:6:1 ratio instead. The empirical formula s atomic ratio is 1:2:1 for C:O:H. However, the acetaldehyde CH 3 CHO has 2:4:1 ratio instead. The empirical formula s atomic ratio is 1:2:1 for C:O:H. However, the benzoic acid C 6 H 5 COOH has 7:6:2 ratio instead. Good job! This is a molecular formula of glucose. The empirical formula s atomic ratio given is 1:2:1 for C:O:H. The glucose C 6 H 12 O 6 has 6:12:6 ratio, i.e. 1:2:1. This matches the multiple of the empirical formula. Therefore glucose is the molecular formula with the empirical formula of CH 2 O. Empirical formula is the lowest possible ratio of atoms in a molecule. The C:H:O ratio is 1:2:1. The molecular formula that is possible would have the same ratio as 1:2:1. Sum up the total of each of three elements. Exam each option, C 6 H 12 O 6 is the only one with 1:2:1 ratio for C:H:O. The rest of the molecules listed do not fit the empirical formula of CH 2 O.

9 No. 9 of Carbon monoxide is a colorless, odorless and tasteless gas, but it is toxic to human and animals. What is the volume of grams of carbon monoxide? (A) 200 L (B) 160 L (C) 714 L (D) 22.4 ml Check your calculation. This needs two-step process. Convert the mass to moles and then moles to volume using the molar volume at STP for gases. B. Correct! Good job! There are two steps to calculate this. Convert the mass to moles and then moles to volume using the molar volume at STP for gases. Check your calculation. This needs two-step process. Convert the mass to moles and then moles to volume using the molar volume at STP for gases. D. Incorrect! Check your calculation. This needs two-step process. Convert the mass to moles and then moles to volume using the molar volume at STP for gases. This is a two-step process. #moles (CO) = Mass/MolecularWeight = (200.0 g)/(28.00g/mol) = 7.14 mol Molar volume Vm is 22.4 L/mol at STP. V(CO) = #Moles x Vm = 7.14 mol x 22.4 L/mol = 160 L. The correct answer is (B).

10 No. 10 of 10 Instruction: (1) Read the problem and answer choices carefully (2) Work the problems on paper as 10. Dinitrogen monoxide or nitrous oxide, is better known as laughing gas. It is used as an anesthetic agent in dentistry. What is the mass of 300. liters of dinitrogen monoxide? (A) 300 g (B) g (C) g (D) 589 g Check your calculation. Use two steps to calculate the mass. Convert the number of moles and then convert it to the mass using molecular mass. Check your calculation. Use two steps to calculate the mass. Convert the number of moles and then convert it to the mass using molecular mass. Check your calculation. Use two steps to calculate the mass. Convert the number of moles and then convert it to the mass using molecular mass. Good job! Use two steps to calculate the mass. Convert the number of moles and then convert it to the mass using molecular mass. Notice the number of significant figures in 300. L. This requires two steps. Be aware of the significant figures in calculation. The molar volume is 22.4 L/mol at STP. With 300 liters of N 2 O, the total number of moles = (300. L)/(22.4 L/mol) = 13.4 mol Mass (N 2 O) = Moles x (Molecular Mass) = 13.4 mol x 44.0 g/mol = 589 g There are three significant figures in the original data. Therefore, the answer should have in three significant figures too.

(A) mole (B) 473 mole (C) mole (D) mole (E) The chemical formula is needed to complete this problem.

(A) mole (B) 473 mole (C) mole (D) mole (E) The chemical formula is needed to complete this problem. College Chemistry - Problem Drill 09: The Mole No. 1 of 10 1. Avogadro s number is defined as a number equal to the number of atoms in 12 grams of the carbon-12. This number is 6.02x10 23. This is one