Test Review Unit 3_1_Amount of substance Mole, molar mass and Avogadro s number test

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1 Test Review Unit 3_1_Amount of substance Mole, molar mass and Avogadro s number test (Information extracted from file FQ3eso_t3_1_Amount of substance ) The Mole Read and then answer the Multiple choice exercise: Just like a dozen means twelve things, a mole is a much larger number of things. These things can be atoms, or molecules, or eggs; however, in chemistry, we usually use the mole to refer to the amounts of atoms or molecules. A mole is defined as the number of things equal to the number of atoms in exactly 12 g of carbon-12; equals things. Experimental measurements have determined that this number is very large: 1 mol = things The numerical value of things in a mole is often called Avogadro s number (N A ), which is also known as the Avogadro constant, after Amadeo Avogadro, an Italian chemist who first proposed its importance. One mole of a compound contains the Avogadro's number (6.022 x 1023) of molecules (molecular compound) or formula units (ionic compound). The molar mass of a compound tells you the mass of 1 mole of that substance. In other words, it tells you the number of grams per mole of a compound. The units for molar mass are, therefore, grams/mole. Knowing the molar mass of a substance, we can calculate the number of moles in a certain mass of a substance and vice versa. The molar mass is used as the conversion factor. Mole, molar mass and Avogadro s number test Multiple Choice: Identify the letter of the choice that best completes the statement or answers the question x is called number. a. Avogadro s c. Aristotle s b. Newton s d. Planck s 2. There are x atoms in one of atoms. a. mole c. amu b. Newton d. kilogram 3. The mass in grams of one mole of any pure substance is called its mass

2 a. atomic c. molar b. formula d. molecular 4. If 24 g of carbon is measured on a balance, how many atoms have been indirectly counted? a. 2 c x b. 24 d x = x Which element has a molar mass of 31 g/mole? a. Potassium c. Gallium b. Phosphorus d. Palladium 6. Which is the mass of an atom of calcium? a. 20 amu c. 40 amu b. 20 g d. 40 g 7. The number of atoms in a mole of any pure substance is called a. its atomic number. c. its mass number. b. Avogadro s number. d. its gram-atomic number. 8. What can be said about 1 mol Ag and 1 mol Au? a. They are equal in mass. b. They contain the same number of atoms. c. Their molar masses are equal. d. They have the same atomic mass. 9. An Avogadro s number of any element is equivalent to a. the atomic number of that element. c particles. b. the mass number of that element. d. 12 g of that element The molar mass of an element is the mass of one a. atom of the element. c. gram of the element. b. liter of the element. d. mole of the element. To determine the molar mass of an element, one must know the element s a. Avogadro number. c. number of isotopes. b. atomic number. d. atomic mass. What SI unit is used to measure the number of representative particles in a substance? a. kilogram c. kelvin b. ampere d. mole Avogadro's number of representative particles is equal to one. a. kilogram c. kelvin b. gram d. mole 14. How many moles of tungsten (W) atoms are in atoms of tungsten? a moles c moles

3 b moles d moles How many atoms are in 3.5 moles of arsenic (As) atoms? a atoms c atoms b atoms d atoms The atomic masses of any two elements contain the same number of. a. atoms c. ions b. grams d. milliliters To determine the formula mass of a compound you should a. add up the atomic masses of all the atoms present. b. add up the atomic masses of all the atoms present and divide by the number of atoms present. c. add up the atomic numbers of all the atoms present. d. add up the atomic numbers of all the atoms present and divide by the number of atoms present 18. Which of the following elements exists as a diatomic molecule? a. Neon (Ne) c. Nitrogen (N 2 ) b. Lithium (Li) d. Sulfur (S 8 ) 19. What is true about the molar mass of chlorine gas (Cl 2 )? a. The molar mass is 35.5 g. b. The molar mass is 71.0 g. c. The molar mass is equal to the mass of one mole of chlorine atoms. d. none of the above 20. Avogadro s number of phosphorus (P) atoms would have a mass of a x g c g b x g d g 21. A mole of a chemical substance represents a. the molar mass of that substance expressed in atomic mass units. b. an Avogadro s number of grams of that substance c. 1.0 g of that substance d. None of the above The atomic masses of He and Be are 4.00 and 9.01 amu, respectively. Which of the following statements is true? a. A mole of Be contains more atoms than a mole of He. b. A mole of He is heavier than a mole of Be. c. A mole of Be contains the same number of atoms as a mole of He. d. none of the above Which of the following samples contains the greatest number of atoms? a x C atoms c g of N atoms

4 b. 2.0 g of He atoms d. 4 moles of O atoms Which of the following statements concerning Avogadro s number is correct? a. It has the value 6.02 x b. It denotes the number of molecules in one mole of any molecular substance. c. It is the mass, in grams, of one mole of any substance.. d. It denotes the number of atoms in one mole of any substance. Which of the following samples contains the greatest number of atoms? a. 1 mole of C. c. 3 moles of N b. 2 moles of He d. 4 moles of O Mole, molar mass and Avogadro s number test. Answers 1. a 11. d 21. d 2. a 12. d 22. c 3. c 13. d 23. d 4. d 14. b 24. d 5. b 15. c 25. b 6. c 16. a 7. b 17. a 8. b 18. c 9. c 19. b 10. d 20. d

5 Converting Moles to Number of Atoms or Number of molecules: Propane is a gas used for cooking and heating. How many molecules are there in 2.12 mol of propane (C3H8)? 1.- Analyze: - List the knowns and the unknown. The desired conversion is: moles molecules KNOWNS: number of moles = 2.12 mol C3 H8 1 mol C3 H8 = molecules C3 H8 UNKNOWN: number of molecules C3 H8 2.- Calculate Solve for the unknown. First write the conversion factor to convert from moles to molecules molecules C3H8 1 mol C3H8 (Remember to write the conversion factors so that the unit in the denominator cancels the unit in the numerator of the previous factor.) Multiply the moles of C3H8 by the conversion factors. = 1.28 x molecules C3H8 Do it yourself: - How many atoms are there in 2 moles carbon (C)? = 1.20 x atoms C

6 - How many molecules are present in 2.76 mol of H 2 O? - How many molecules are there in 4.00 moles of glucose, C6H12O6? Molecules= 4.00 moles x10 molecules 1 mole = 2.41 x molecules Converting Number of Atoms or Number of molecules to Moles: - How many moles of magnesium is 3.01 x atoms of magnesium? 1 mole n= 3.01 x atoms x10 atoms = 5 x 10-2 moles The molar mass The molar mass of a compound tells you the mass of 1 mole of that substance. In other words, it tells you the number of grams per mole of a compound. The units for molar mass are, therefore, grams/mole. To find the molar mass of a compound: 1.- Use the chemical formula to determine the number of each type of atom present in the compound. 2.- Multiply the atomic mass (from the periodic table) of each element by the number of atoms of that element present in the compound. 3.- Add it all together and put units of grams/mole after the number. Knowing the molar mass of a substance, we can calculate the number of moles in a certain mass of a substance and vice versa, as these examples illustrate. The molar mass is used as the conversion factor. Calculate the relative molecular mass of each of the following: (a) H 2 S (b) CH 4 (c) NH 3 (d) C 4 H 10 (e) C 6 H 12 O 6 (f) C 12 H 22 O 11 (g) C 2 H 5 OH. Answers: (a) M r (H 2 S) = 2 X M r (H) + M r (S) =2 X = = 34 amu

7 (b) M r (CH 4 ) = M r (C) + 4 X M r (H) = X 1 = = 16 amu (c) M r (NH 3 ) = M r (N) + 3 X M r (H) = X 1 = = 17 amu (d) M r (C 4 H 10 ) = 4 X M r (C) + 10 X M r (H) = 4 X X 1 = = 58 amu (e) M r (C 6 H 12 O 6 ) = 6 X M r (C) + 12 X M r (H) + 6 X M r (O) = 6 X X X 16 = = 180 amu (f) M r (C 12 H 22 O 11 ) = 12 X M r (C) + 22 X M r (H) + 11 X M r (O) = 12 X X X 16 = = 342 amu (g) M r (C 2 H 5 OH) = 2 X M r (C) + 5 X M r (H) + M r (O) + M r (H) = 2 X X = = 46 amu Mole-Mass Conversions 1. How many moles in 28 grams of CO2? Gram-formula-mass of CO g/mol 1 mole 28 g CO 2 = 0.64 moles CO g 2 2. What is the mass of 5 moles of Fe2O3? Gram-formula-mass Fe 2 O g/mol g 5 moles Fe 2 O 3 = 800 grams Fe 1 mole 2 O 3 What is the mass of 3.56 mol of HgCl 2? The molar mass of HgCl 2 is g/mol.