Exam 1, SPRING 2000

White Copy: 1. Pure substances must be either and element or a compound. The only other options would be mixtures but by definition a mixture is not a pure substance. 2. The three phases of matter are solid, liquid, and gas. 3. Pure substances which are composed of more than one atom chemically bonded together are: B) compounds The key is bonded together. Single atoms are elements. Mixtures have no bonds between the different parts. 4. A mixture in which the composition varies throughout is a: C) heterogeneous mixture Hetero is a greek term meaning "different". If the composition varies then the composition is "different". 5. The SI unit of length is the: D) meter 6. Which are the SI units of temperature, pressure, and mass, respectively? B) kelvin degrees, pascals, kilograms 7. Which of the following measurements contains four significant figures? A) 5.115 cm 8. Which of the following measurements is MOST precise? A) 3.1416 kg Precision comes from measuring more closely so the more digits the more precise. 9. What is the result, with the proper number of significant figures, when 6.005 m is multiplied by 22.31 m? D) 134.0 m^{2}The result of the math is 133.97 so one needs to round off first to 134. But we need 4 significant figures and thus add a trailing zero with a decimal point to show precision. 10. Convert 224.0 mL to liters. 1 L 224.0 mL x --------- = 0.224 L 1000 mL 11. Convert 157 C to Kelvin To convert from C to K requires adding 273, so the answer is 430 K 12. Water has a density of 1.00 g/cm3. What is the mass, in kilograms, of 1.00 m^{3}of water? g 100 cm 100 cm 100 cm 1 kg 1.00 --- x ------ x ------ x ------ x ------ = 1 x 10^{3}kg cm3 1 m 1 m 1 m 1000 g 13. The solubility of potassium chloride (should be bromide), KBr, at 70 C is 48.3 g/100 g of water. Calculate the mass of KBr which will dissolve in 250g of water at 70 C. 48.3 g X g ------ = ------- 100 g 250 g Cross multiply and solve for X. X = 121 g = 1.21 x 10^{2}g 14. At the melting point of a pure solid, B) solid is changing to liquid 15. The nucleus of a lead atom,^{207}_{82}Pb, contains: 82 protons 'cause that's what the atomic number means and 207 - 82 = 125 neutrons because the atomic mass is the sum of the masses of the protons plus the neutrons. 16. What is the symbol of an ion which has 20 protons and 18 electrons? 20 protons is Calcium, symbol, Ca Since the # of electrons is less than the # of protons, the ion has a positive charge. The difference is 2 so the proper symbol for the ion is: Ca^{2+}17. The periodic table is arranged by: the number of protons in the nucleus. 18. Skipped nothing like this on final. 19. Name the elements whose symbols are K P S Na potassium phosphorus sulfur sodium 20. Name this compound: CaS calcium sulfide It is a sulfide because there is no oxygen with the sulfur. The anion receives the -ide suffix when no oxygen is present. 21. The mass of Sr, strontium, is listed as 87.62. Which answer best explains why the mass is not a whole number? D) the mass shown in the periodic table is an average mass based on natural isotopes. 22. Which of the following contains the same number of electrons as neutrons? The answer is A because: Chlorine has 17 protons and 18 neutrons in the atomic state. For the number of electrons to equal the number of neutrons, chlorine must gain one electron. Thus Cl^{-}has 18 electrons which equals the number of neutrons. Answer B has 17 electrons and 18 neutrons, so they are not equal. Answer C has 18 electrons and 16 neutrons, again not equal. Answer D has 18 electrons and 22 neutrons. Not equal. Answer E has 15 electrons and 16 neutrons. Not equal. 23. Which of the following contains more electrons than neutrons? See above for counts. The answer is C. 24. From the decomposition of a pure solid, we obtain a solid and a gas, each of which is a pure substance. We can conclude that: A) Correct, elements cannot be decomposed. B) WRONG, we said it was pure. C) WRONG, there are many pure substances which are solids or gases. D) WRONG, there are many pure substances which are solids and gases which are not elements. E) WRONG, there are solid elements and gaseous compounds. This was an effort in critical thinking. 25. Which of the following lists calcium, carbon, chlorine, sulfur, and silicon in the same order as these names? B) Ca, C, Cl, S, Si 26. The correct formula for potassium phosphate is: Potassium is in Group 1 so it carries a charge of +1. Phosphate ion is PO_{4}^{3-}. To form a compound, the charges must balance. So we need 3 + to balance 3 -. D) K_{3}PO_{4}27. In Al_{2}O_{3}, what is the charge on the oxygen ion? Oxygen ion ALWAYS has a charge of 2-. The question was NOT plural "ions". 28. Which of the following elements has the largest atomic mass? Look in the periodic table. B) tin (Sn) 29. How many elements are there in Group 16 of the periodic table? C) 5, just count down the column. 30. Magnesium metal is reacted with oxygen gas and a white powder, magnesium oxide, is produced. If 3.56 g of Mg reacts with 7.00 g of oxygen to produce 5.93 g of magnesium oxide and some unreacted oxygen, what is the mass of oxygen that has reacted with magnesium? B) 2.37 g 31. What is the formula of the hydroxide ion? A) OH^{-}32. What is the formula of iron(III) chloride? The III means the Fe has a charge of 3+. Chloride has a charge of 1- so it takes 3 Cl^{-}to balance one Fe^{3+}. B) FeCl_{3}33. The formula of acetic acid is CH_{3}CO_{2}H. How many carbon atoms are in on acetic acid molecule? Just count how many are in the formula since formulas describe the composition of the molecule. B) 2 34. How many moles of hydrogen atoms are in 0.50 mol of acetic acid, CH_{3}CO_{2}H? The are 4 hydrogen atoms in each acetic acid molecule. Since moles are molecules times Avogadro's Number, the proportion stays the same. Thus 4 x 0.50 mol = 2 mol 35. What is the molar mass of acetic acid, CH_{3}O_{2}H? 2 x 12 = 24 4 x 1 = 4 16 x 2 = 32 ---- total = 60 The Atomic Numbers represent amu values for a molecule and g measurements for moles so the answer is: D) 60 g/mol 36. What is the mass in grams of 1.0 x 108 atoms of iron (Fe)? The mass of 1 atom is: 55.847 g ----------- = 9.28 x 10^{-23}6.02 x 10^{23}because there are Avogadro's Number of atoms in a mole of Fe (or any substance) and the mass of one mole of Fe is 55.847 g. The mass of 1.0 x 10^{8}atoms is then: 1.0 x 10^{8}x 9.28 x 10^{-23}= 9.29 x 10^{-15}g The answer is D 37. A tablet of aspirin contains 0.500 g of aspirin (molar mass = 180 g/mol). How many moles of aspirin are in the tablet? Ratio: 0.500 g 180 g --------- = --------- X mol 1 mol 180X = 0.5 X = 0.00278 mol The answer is C 38. What is the mass in grams of a single molecule of water? The mass of a mole of water is 18.02 g. There are Avogadro's Number of molecules in a mole of water. We want the mass in one molecule so: 18.02 g -------------- = 2.99 x 10-23 g/molecule 6.02 x 10^{23}The answer is D 39. What is the mass of 5.00 millimoles of NaCl (58.44 g/mol)? A millimole may seem confusing BUT remember milli = 10^{-3}so a millimole is 10^{-3}mole. 5.00 millimoles converts to 5.00 x 10^{-3}mol Next multiply by the mass of one mole: 5.00 x 10^{-3}mol x 58.44 g/mol 0.292 g 40. Ammonium nitrate (NH_{4}NO_{3}) is used as a fertilizer. How many grams of nitrogen atoms are in 1.00 mol of NH_{4}NO_{3}? There are 2 N in each NH_{4}NO_{3}so: 2 x 1.00 mol = 2 mol of N Molar mass of N is 14 g/mol so: 2 mol N x 14 g/mol = 28 g N