Chem Unit 1 Practice Problems- AP CLASSROOM

How many carbon atoms are contained in 2.8 g of C2H4 ? A. 1.2 x 10^23 B. 3.0 x 10^23 C. 6.0 x 10^23 D. 1.2 x 10^24 E. 6.0 x 10^24

A. 1.2 x 10^23

A student was studying physical and chemical changes. The student carried out some procedures in the laboratory and recorded observations. For one of the procedures, the student concluded that a physical change took place, but not a chemical change. Which of the following could have been the results of the procedure? A. A cube of metal was changed into a flat sheet of metal. B. When two liquids at room temperature were combined in a beaker, the beaker became hot. C. When two clear liquids were combined, the resulting mixture was cloudy. D. When a colorless liquid was added to a blue liquid, the resulting solution was yellow.

A. A cube of metal was changed into a flat sheet of metal.

C6H12O6+6O2→6CO2+6H2O The reaction between C6H12O6 and O2 is represented by the balanced equation above. In an experiment, 0.30mol of CO2 was produced from the reaction of 0.05mol of C6H12O6 with excess O2. The reaction was repeated at the same temperature and in the same container, but this time 0.60mol of CO2 was produced. Which of the following must be true? A. The initial amount of C6H12O6 in the container must have been 0.10mol. B. Exactly 0.30mol of C6H12O6 must have reacted because C atoms were conserved. C. Exactly 0.40mol of O2 of must have reacted because the temperature and container volume are the same. D. More than 0.60mol of O2 must have reacted because it was present in excess.

A. The initial amount of C6H12O6 in the container must have been 0.10mol. There are 1.80mol of C atoms in 0.30mol of C6H12O6, but there is only 0.60mol of C atoms in the 0.60mol of CO2 that was produced. Since there are fewer C atoms in the product, C atoms were not conserved. To produce 0.60mol of CO2, 0.10mol of C6H12O6 must have reacted because the ratio CO2:C6H12O6 is 6:1 in the balanced equation.

The mass percent of carbon in pure glucose, C6H12O6, is 40.0 percent. A chemist analyzes an impure sample of glucose and determines that the mass percent of carbon is 38.2 percent. Which of the following impurities could account for the low mass percent of carbon in the sample? A. Water, H2O B. Ribose, C5H10O5 C. Fructose, C6H12O6, an isomer of glucose D. Sucrose, C12H22O11

A. Water, H2O

A student has samples of two pure compounds, XClO3 and ZClO3, which contain unknown alkali metals X and Z. The student measures the mass of each sample and then strongly heats the samples to drive off all the oxygen, leaving solid residues of XCl and ZCl. The student measures the mass of the solid residue from each sample. Which of the following questions can be answered from the results of the experiment? A. Which has the greater molar mass, X or Z? B. Which has the higher boiling point, X or Z? C. Which has the higher melting point, XCl or ZCl? D. Which has the greater density, XCl or ZCl?

A. Which has the greater molar mass, X or Z?

A student has two samples of NaCl, each one from a different source. Assume that the only potential contaminant in each sample is KCl. The student runs an experiment to determine the percent by mass of chlorine in each sample. From the results of this experiment alone, which of the following questions is most likely to be answered? A. Which sample has the higher purity? B. Which sample has the higher density? C. What is the source of the contaminants present in each of the samples? D. Which sample came from a salt mine, and which sample came from the ocean?

A. Which sample has the higher purity?

2F2(g)+2NaOH(aq)→OF2(g)+2NaF(aq)+H2O(l) A 2mol sample of F2(g) reacts with excess NaOH(aq) according to the equation above. If the reaction is repeated with excess NaOH(aq) but with 1 mol of F2(g), which of the following is correct? A. The amount of OF2(g)OF2(g) produced is doubled. B. The amount of OF2(g)OF2(g) produced is halved. C. The amount of NaF(aq)NaF(aq) produced remains the same. D. The amount of NaF(aq)NaF(aq) produced is doubled.

B. The amount of OF2(g)OF2(g) produced is halved.

A student is given two 10g samples, each a mixture of only NaCl(s) and KCl(s) but in different proportions. Which of the following pieces of information could be used to determine which mixture has the higher proportion of KCl(s) ? A. The volume of each mixture B. The mass of Cl in each mixture C. The number of isotopes of Na and K D. The reaction of each mixture with water

B. The mass of Cl in each mixture

HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l) A student had two dilute, colorless solutions, HCl(aq) and NaOH(aq), which were at the same temperature. The student combined the solutions, and the reaction represented above occurred. Which of the following results would be evidence that a chemical reaction took place? A. The resulting solution is colorless. B. The temperature of the reaction mixture increases. C. The total volume of the mixture is approximately equal to the sum of the initial volumes. D. The resulting solution conducts electricity.

B. The temperature of the reaction mixture increases.

Two different ionic compounds each contain only copper and chlorine. Both compounds are powders, one white and one brown. An elemental analysis is performed on each powder. Which of the following questions about the compounds is most likely to be answered by the results of the analysis? A. What is the density of each pure compound? B. What is the formula unit of each compound? C. What is the chemical reactivity of each compound? D. Which of the two compounds is more soluble in water?

B. What is the formula unit of each compound?

A student was asked to formulate a hypothesis about what would happen if 100.mL of 0.1MNaOH(aq) at 25°C was combined with 100.mL of 0.1MMgCl2(aq) at 25°C. Which of the following hypotheses indicates that the student thought a chemical change would occur? A. The volume of the resulting solution will be equal to the sum of the volumes of the original solutions. B. The mass of the resulting solution will be equal to the sum of the masses of the original solutions. C. The resulting solution would contain a precipitate. D. The resulting solution will be clear.

C. The resulting solution would contain a precipitate.

A student mixes 20.0g of white KCl crystals with distilled water in a beaker. After the mixture was stirred, no crystals are visible and the solution is clear. After several days, all of the water evaporates and white crystals are found in the beaker. Which of following pieces of experimental evidence would best help the student to confirm that a new compound had not been made and that only a physical change occurred? A. The solution does not change color after stirring. B. The KCl crystals are no longer visible after mixing with water. C. There is a temperature change in the solution during the dissolving process. D. After the water has evaporated, the white crystals in the beaker have a mass of 20.0g.

D. After the water has evaporated, the white crystals in the beaker have a mass of 20.0g.

A student has a 1g sample of each of the following compounds: NaCl, KBr, and KCl. Which of the following lists the samples in order of increasing number of moles in the sample? A. NaCl<KCl<KBrNaCl<⁢KCl⁢<KBr B. NaCl<KBr<KClNaCl<KBr⁢<KCl C. KCl<NaCl<KBrKCl<NaCl⁢<KBr D. KBr<KCl<NaCl

D. KBr<KCl<NaCl

MgCl2(aq)+2NaOH(aq)→2NaCl(aq)+Mg(OH)2(s) A 100mL sample of 0.1MMgCl2(aq) and a 100mL sample of 0.2MNaOH(aq) were combined, and Mg(OH)2(s) precipitated, as shown by the equation above. If the experiment is repeated using solutions of the same molarity, which of the following changes in volume will double the amount of Mg(OH)2(s) produced? A. Using the same volume of MgCl2(aq) but twice the volume of NaOH(aq) B. Using twice the volume of MgCl2(aq) but half the volume of NaOH(aq) C. Using twice the volume of MgCl2(aq) but the same volume of NaOH(aq) D. Using twice the volume of MgCl2(aq) and twice the volume of NaOH(aq)

D. Using twice the volume of MgCl2(aq) and twice the volume of NaOH(aq)

Given that the density of Hg(l) at 0°C is about 14 g mL-1, which of the following is closest to the volume of one mole of Hg(l) at this temperature? a. 0.070 mL B. 0.14 mL C. 1.4 mL D. 14 mL E. 28 mL

a. 0.070 mL