2 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 2
Which of the numbered diagrams represents a pure substance?
Answer and reasoning
ADiagram 1 A student who judges purity by the kinds of atoms present picks this. The box contains only C and O atoms, but in two types of molecules, CO and CO₂, so it is a mixture.
BDiagram 2 A student who thinks elements mixed in the ratio of a formula are the compound picks this. The box holds one C atom for every two O atoms, as in CO₂, but the C atoms and O₂ molecules are not bonded together; the box is a mixture of two elements.
CDiagram 3 A student who thinks an evenly mixed sample is pure picks this. The N₂ and O₂ molecules are spread uniformly, but there are two types of molecules, so this is a homogeneous mixture.
DDiagram 4Correct Every particle in the box is the same: a CO₂ molecule. A sample containing molecules of a single type is a pure substance; the other boxes each contain two types of particles.
A 2.00 g sample of impure potassium chloride, KCl, is found by elemental analysis to contain 0.855 g of chlorine. The impurity contains no chlorine. In addition to these data, which information does a student need in order to calculate the percent purity of the KCl?
Answer and reasoning
AThe mass percent of Cl in KCl, from molar massesCorrect The chlorine is part of the KCl, so its mass must be converted to the mass of KCl: mass of KCl = 0.855 g ÷ (35.45/74.55) = 1.80 g. The purity is then 1.80/2.00 = 89.9%.
BNone; the percent purity is 0.855 g ÷ 2.00 g × 100% A student who takes the mass of chlorine to be the mass of KCl picks this. 0.855 g ÷ 2.00 g gives the mass percent of chlorine in the sample (42.8%), not of KCl; the KCl also contains potassium.
CThe number of atoms in one formula unit of KCl A student who thinks the mass fraction of an element equals its fraction of the atoms picks this. Cl is half of the atoms in KCl but 47.55% of its mass; the molar masses, not the number of atoms, are needed.
DThe identity and the molar mass of the impurity A student who thinks the impurity must always be identified picks this. The impurity contains no chlorine, so all the chlorine is in KCl, and the mass of KCl can be found without knowing what the impurity is.
Working Mass fraction of Cl in KCl = 35.45/74.55 = 0.4755. Mass of KCl = 0.855 g ÷ 0.4755 = 1.80 g. Purity = 1.80/2.00 × 100% = 89.9%. The quantity needed is the mass percent of Cl in pure KCl (from the molar masses).
In preparation: 0 of 2 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
1.4.A.1 Pure substance Fix
Pure substance
A sample containing atoms, molecules or formula units of a single type. An element (such as O₂) or a compound (such as CO₂) is a pure substance with a fixed composition.
Mixture
A sample containing atoms, molecules or formula units of two or more types that are not chemically combined. The relative proportions of the components can vary from sample to sample.
Homogeneous mixture
A mixture whose composition is uniform throughout, such as air or a salt solution. Being uniform does not make it a pure substance: it still contains more than one kind of particle in proportions that can vary.
Mass percent of an element in a mixture
The mass of the element divided by the total mass of the mixture, times 100%. Unlike the percent composition of a compound, it changes when the proportions of the components change.
Students often think Purity depends on the kinds of atoms present, so a sample containing only one set of elements (or only one element) is a pure substance even if it contains different molecules. In fact No. A pure substance contains only one type of particle. A sample of CO and CO₂ molecules contains only carbon and oxygen atoms, but it has two types of molecules, so it is a mixture.
Students often think A compound is just its elements mixed in the right proportions, so elements mixed in the ratio of a formula are the same as the compound. In fact No. Carbon dioxide is a compound in which each C atom is bonded to two O atoms. Separate C atoms and O₂ molecules, even in the right ratio, are a mixture of two elements until they react.
1.4.A.2 Elemental analysis Fix
Elemental analysis
Measurement of the mass (or mass percent) of one or more elements in a sample. Combined with the formula of a compound, it gives the relative numbers of atoms in the substance or the amount of that compound in the sample.
Percent purity
The mass of the desired compound in a sample as a percentage of the sample's mass. It can be found from elemental analysis when only the desired compound contains the element measured: mass of compound = mass of element ÷ mass fraction of the element in the compound.
Students often think The mass of an element found by analysis is the mass of the compound that contains it, so percent purity is the mass of the element divided by the mass of the sample. In fact No. An element is only part of the mass of a compound: the mass of compound = mass of element ÷ mass fraction of the element in the compound (for KCl, 0.4755).
Students often think The mass fraction of an element in a compound equals its fraction of the atoms in the formula, so the number of atoms per formula unit is what converts an element's mass to the compound's mass. In fact No. Atoms of different elements have different masses: Cl is half of the atoms in NaCl but 60.66% of its mass, and K is half of the atoms in KCl but 52.45% of its mass.
4 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 4
A mixture of 1.00 g of NaCl and 4.00 g of sand, which contains no chlorine, is 12.1% chlorine by mass. Another 1.00 g of NaCl is added to the mixture and mixed in. What is the mass percent of chlorine in the new mixture?
Answer and reasoning
A12.1% A student who thinks a mixture has a fixed composition picks this. Adding NaCl changes the proportions: the mixture now contains 1.213 g of chlorine in 6.00 g, or 20.2%.
B20.2%Correct NaCl is 60.66% chlorine by mass, so 2.00 g of NaCl contains 1.213 g of Cl. The new mixture has a total mass of 6.00 g, so it is 1.213/6.00 = 20.2% chlorine by mass.
C24.2% A student who thinks doubling the NaCl doubles the mass percent of chlorine picks this. The mass of chlorine doubles, but the total mass also increases, from 5.00 g to 6.00 g, so the percentage rises only to 20.2%.
D60.7% A student who gives the mixture the composition of pure NaCl picks this. The sand adds 4.00 g of mass but no chlorine, so the mixture is only 20.2% chlorine.
Working Mass fraction of Cl in NaCl = 35.45/58.44 = 0.6066. New mixture: 2.00 g NaCl contains 2.00 × 0.6066 = 1.213 g Cl; total mass = 2.00 + 4.00 = 6.00 g. Mass percent Cl = 1.213/6.00 × 100% = 20.2%.
A 2.50 g sample of a mixture of sodium chloride, NaCl, and sand is found to contain 1.00 g of chlorine. The sand contains no chlorine. What is the percentage by mass of NaCl in the sample?
Answer and reasoning
A24.3% A student who applies the mass ratio upside down picks this, multiplying 1.00 g by 35.45/58.44 to get 0.607 g of NaCl. That is less than the mass of chlorine it contains, which is impossible; the factor is 58.44 g NaCl/35.45 g Cl.
B40.0% A student who takes the mass of chlorine as the mass of NaCl picks this. 40.0% is the mass percent of chlorine in the sample; the sodium in the NaCl must be included.
C65.9%Correct 58.44 g of NaCl contains 35.45 g of Cl, so 1.00 g of Cl is in 1.00 × 58.44/35.45 = 1.649 g of NaCl. 1.649 g/2.50 g = 65.9% NaCl by mass.
D80.0% A student who takes the mass fraction of chlorine in NaCl to be its atom fraction, one half, picks this, doubling 1.00 g to 2.00 g of NaCl. Chlorine is 60.66% of the mass of NaCl, not 50%.
Working Mass of NaCl = 1.00 g Cl × (58.44 g NaCl/35.45 g Cl) = 1.649 g. Percentage of NaCl = 1.649 g/2.50 g × 100% = 65.9%.
A student finds the purity of a sample of KCl by measuring the mass of chlorine in it and assuming that all of the chlorine is in KCl. Unknown to the student, the only impurity in the sample is NaCl. Data for the two compounds are given in the table. How does the percent purity the student calculates compare with the actual percentage of KCl in the sample?
Answer and reasoning
AIt is higher, because each gram of NaCl is counted as more than one gram of KClCorrect The student's calculation treats all of the measured chlorine as if it were in KCl. NaCl is 35.45/58.44 = 60.66% chlorine and KCl is 35.45/74.55 = 47.55% chlorine, so each gram of NaCl supplies 0.6066 g of chlorine, which the student counts as 0.6066/0.4755 = 1.28 g of KCl. The calculated mass of KCl, and the calculated purity, are too high.
BIt is equal, because the mass of chlorine measures the KCl whatever else is present A student who thinks the mass of an element measures the desired compound whatever else is present picks this. The NaCl also contains chlorine, which the student wrongly counts as part of KCl.
CIt is lower, because any impurity in a sample makes the calculated purity too low A student who thinks every impurity lowers the calculated purity picks this. This impurity contains chlorine, which is counted as if it were in KCl, so the calculated purity is too high.
DIt is higher, because each gram of NaCl is counted as under one gram of KCl A student who thinks a smaller molar mass means a smaller percentage of chlorine picks this, reasoning that NaCl supplies less chlorine per gram than KCl. One mole of each compound contains 35.45 g of Cl, so NaCl, with the smaller molar mass, has the greater mass percent of chlorine (35.45/58.44 = 60.66% against 35.45/74.55 = 47.55%), and each gram of it is counted as 0.6066/0.4755 = 1.28 g of KCl.
A mixture of two solids is 40.0% NaCl and 60.0% KCl by mass. What is the mass percent of chlorine in the mixture?
Answer and reasoning
A47.6% A student who gives a mixture the percent composition of a compound in it picks this, the mass percent of chlorine in KCl, the main component. The 40.0% of the mixture that is NaCl is richer in chlorine (60.66%), so the mixture contains a greater percentage of chlorine than pure KCl.
B50.0% A student who takes the mass fraction of an element to be its fraction of the atoms picks this, because half of the atoms in NaCl and half of the atoms in KCl are Cl. A Cl atom does not have the same mass as an Na atom or a K atom: chlorine is 60.66% of the mass of NaCl and 47.55% of the mass of KCl.
C52.8%Correct NaCl is 35.45/58.44 = 60.66% chlorine and KCl is 35.45/74.55 = 47.55% chlorine. Each compound contributes in proportion to its share of the mixture: 0.400 × 60.66% + 0.600 × 47.55% = 24.3% + 28.5% = 52.8%.
D54.1% A student who takes the simple average of the two compounds' chlorine percentages picks this: (60.66% + 47.55%)/2. That would be right for equal masses of the two compounds; this mixture contains more KCl than NaCl, so its percentage of chlorine is closer to that of KCl.
Working Mass fraction of Cl in NaCl = 35.45/58.44 = 0.6066; in KCl = 35.45/74.55 = 0.4755. In 100.0 g of mixture: 40.0 g NaCl contains 40.0 × 0.6066 = 24.26 g Cl and 60.0 g KCl contains 60.0 × 0.4755 = 28.53 g Cl. Total Cl = 52.8 g in 100.0 g, so the mixture is 52.8% chlorine by mass.
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
