When the equation C3H8 + O2 -> CO2 + H2O for the combustion of propane is balanced with the smallest whole-number coefficients, what is the coefficient of O2?
5
✓
3
4
10
Correct Answer
5
Balancing carbon gives 3 CO2 and balancing hydrogen gives 4 H2O, which together contain 6 + 4 = 10 oxygen atoms, so 5 O2 are needed: C3H8 + 5O2 -> 3CO2 + 4H2O. 3 is the coefficient of CO2. 4 is the coefficient of H2O. 10 is the number of oxygen atoms, not the number of O2 molecules.
Category: Balancing Equations
Question 2
A chemical equation is balanced by adjusting which of the following?
Subscripts within the formulas
Coefficients in front of formulas
✓
Charges written on the ions
Identities of the products
Correct Answer
Coefficients in front of formulas
Coefficients change how many units of each substance take part, which balances atoms without changing what the substances are. Subscripts within the formulas cannot be changed, because that would turn a substance into a different compound. Charges written on the ions are fixed by the identity of each ion. Identities of the products are set by the actual chemistry and cannot be altered to make the numbers work.
Category: Balancing Equations
Question 3
For the reaction AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq), which ions are spectator ions?
Ag+ and Cl-
Na+ and Cl-
Na+ and NO3-
✓
Ag+ and NO3-
Correct Answer
Na+ and NO3-
Na+ and NO3- are dissolved on both sides of the equation and do not change, so they are left out of the net ionic equation, Ag+(aq) + Cl-(aq) -> AgCl(s). Ag+ and Cl- are the ions that combine to form the solid precipitate. Na+ and Cl- includes chloride, which leaves solution as part of AgCl. Ag+ and NO3- includes silver, which also ends up in the precipitate.
Category: Ionic Equations
Question 4
What is the net ionic equation for the reaction of aqueous HCl with aqueous NaOH?
Na+(aq) + Cl-(aq) -> NaCl(s)
HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)
Na+(aq) + OH-(aq) -> NaOH(aq)
H+(aq) + OH-(aq) -> H2O(l)
✓
Correct Answer
H+(aq) + OH-(aq) -> H2O(l)
HCl is a strong acid and NaOH is a strong base, so both exist as ions; after the spectator ions Na+ and Cl- are removed, only H+(aq) + OH-(aq) -> H2O(l) remains (it may also be written with H3O+ forming two H2O). Na+(aq) + Cl-(aq) -> NaCl(s) is wrong because sodium chloride is soluble and stays dissolved. HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l) is the molecular equation, not the net ionic equation. Na+(aq) + OH-(aq) -> NaOH(aq) shows no actual change, since NaOH is already dissociated.
Category: Ionic Equations
Question 5
Which sulfate is insoluble in water?
BaSO4
✓
Na2SO4
K2SO4
MgSO4
Correct Answer
BaSO4
Sulfates are generally soluble, but those of Ag+, Ba2+, Ca2+, Hg2 2+, Pb2+, and Sr2+ are exceptions, so BaSO4 is insoluble. Na2SO4 is soluble because sodium is a Group 1 cation. K2SO4 is soluble for the same reason. MgSO4 is soluble because magnesium is not among the sulfate exceptions.
Category: Precipitation
Question 6
Aqueous solutions of Pb(NO3)2 and KI are mixed. Which solid precipitates?
KNO3
PbI2
✓
KI
Pb(NO3)2
Correct Answer
PbI2
The ions exchange partners, and iodides of Pb2+ are an exception to the rule that iodides are soluble, so yellow PbI2 precipitates. KNO3 stays dissolved, because nitrates and Group 1 compounds are soluble. KI is one of the soluble starting materials. Pb(NO3)2 is also a soluble starting material, since all nitrates are soluble.
Category: Precipitation
Question 7
Which of these acids is a strong acid?
HF
HC2H3O2
HNO3
✓
H2CO3
Correct Answer
HNO3
Nitric acid, HNO3, is one of the common strong acids, along with HCl, HBr, HI, HClO4, and H2SO4, and it reacts completely with water. HF is a weak acid despite fluorine's high electronegativity. HC2H3O2, acetic acid, is a weak acid that reacts only partly with water. H2CO3, carbonic acid, is also a weak acid.
Category: Acids and Bases
Question 8
Which substance is a weak base that reacts only partly with water to produce OH- ions?
NaOH
KOH
Ba(OH)2
NH3
✓
Correct Answer
NH3
Ammonia, NH3, is a weak base: only a small fraction of the dissolved NH3 molecules accept H+ from water, forming NH4+ and OH-. NaOH is an ionic hydroxide that releases its OH- completely, making it a strong base. KOH is likewise a strong base. Ba(OH)2 is also an ionic hydroxide and a strong base.
Category: Acids and Bases
Question 9
In a combustion analysis, a sample produces 4.40 g of CO2. Using CO2 = 44.01 g/mol and C = 12.01 g/mol, what mass of carbon was in the sample, to three significant figures?
1.20 g
✓
4.40 g
3.20 g
0.100 g
Correct Answer
1.20 g
All of the carbon in the sample ends up in CO2, so the carbon mass is 4.40 g x (12.01 / 44.01) = 1.20 g. 4.40 g is the mass of the whole CO2, which also includes the oxygen that combined with the carbon during burning. 3.20 g is the mass of the oxygen in the CO2. 0.100 g is the number of moles of CO2 reported as if it were a mass.
Category: Chemical Analysis
Question 10
What is the oxidation number of sulfur in the sulfate ion, SO4 2-?
+4
+6
✓
-2
+8
Correct Answer
+6
Each oxygen is -2, and the oxidation numbers must add to the ion's charge, so S + 4(-2) = -2 and S = +6. +4 is sulfur's oxidation number in sulfite, SO3 2-. -2 is the value for sulfur in a sulfide such as Na2S. +8 results from ignoring the 2- charge and setting the sum to zero.
Category: Oxidation Numbers
Question 11
What is the oxidation number of hydrogen in sodium hydride, NaH?
+1
-1
✓
+2
Correct Answer
-1
Hydrogen is usually +1 when bonded to nonmetals, but it is -1 when combined with a metal, as in NaH, where sodium is +1. +1 is hydrogen's usual value in compounds with nonmetals, such as H2O. 0 applies only to elemental hydrogen, H2. +2 would require hydrogen to lose two electrons, but it has only one.
Category: Oxidation Numbers
Question 12
In the reaction Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s), which species is the oxidizing agent?
Zinc metal
Zinc ion
Copper metal
Copper(II) ion
✓
Correct Answer
Copper(II) ion
The copper(II) ion gains two electrons and is reduced to copper metal, and the species that is reduced is the oxidizing agent. Zinc metal loses electrons, so it is oxidized and acts as the reducing agent. Zinc ion is a product formed by that oxidation. Copper metal is a product formed by the reduction.
Category: Redox Reactions
Question 13
In a redox reaction, the species that is oxidized
Loses electrons, raising its oxidation number
✓
Gains electrons, raising its oxidation number
Loses electrons, lowering its oxidation number
Gains electrons, lowering its oxidation number
Correct Answer
Loses electrons, raising its oxidation number
Oxidation is the loss of electrons, and losing negative charge makes the oxidation number increase. Gains electrons, raising its oxidation number pairs the wrong process with the change. Loses electrons, lowering its oxidation number has the direction of the change reversed. Gains electrons, lowering its oxidation number describes reduction.
Category: Redox Reactions
Question 14
The reaction Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g) is best classified as which type of reaction?
Precipitation
Single displacement
✓
Acid-base neutralization
Combustion
Correct Answer
Single displacement
Magnesium replaces hydrogen in the acid, so this is a single-displacement reaction, which is a kind of redox reaction: Mg goes from 0 to +2 and H goes from +1 to 0. Precipitation is wrong because no solid forms; MgCl2 is soluble. Acid-base neutralization is wrong because magnesium metal is not a base and no water forms. Combustion is wrong because there is no fuel reacting with oxygen.
Category: Redox Reactions
Question 15
For the reaction N2 + 3H2 -> 2NH3, how many moles of NH3 form from the complete reaction of 7.5 mol of H2 with enough N2?
11 mol
2.0 mol
5.0 mol
✓
15 mol
Correct Answer
5.0 mol
The balanced equation gives 2 mol NH3 for every 3 mol H2, so 7.5 mol H2 x (2/3) = 5.0 mol NH3. 11 mol uses the mole ratio upside down (7.5 x 3/2). 2.0 mol is the coefficient of NH3, not the amount formed from 7.5 mol of hydrogen. 15 mol multiplies by 2 and ignores the 3 in the ratio.
Category: Reaction Stoichiometry
Question 16
For the reaction 2H2 + O2 -> 2H2O, what mass of water forms when 4.0 g of H2 reacts completely? Use H2 = 2.0 g/mol and H2O = 18.0 g/mol.
18 g
72 g
4.0 g
36 g
✓
Correct Answer
36 g
First convert to moles: 4.0 g / 2.0 g/mol = 2.0 mol H2. The 2:2 mole ratio gives 2.0 mol H2O, and 2.0 mol x 18.0 g/mol = 36 g. 18 g assumes only one mole of water forms. 72 g doubles the amount, as if each H2 made two water molecules. 4.0 g wrongly assumes the product has the same mass as the hydrogen alone, ignoring the oxygen.
Category: Reaction Stoichiometry
Question 17
For the reaction 2Na + Cl2 -> 2NaCl, a mixture contains 7.0 mol of Na and 3.0 mol of Cl2. What is the maximum amount of NaCl that can form?
6.0 mol
✓
7.0 mol
10. mol
3.0 mol
Correct Answer
6.0 mol
3.0 mol Cl2 needs 6.0 mol Na, and 7.0 mol is available, so Cl2 runs out first and limits the product to 2 x 3.0 = 6.0 mol NaCl, leaving 1.0 mol Na in excess. 7.0 mol is based on the Na, which is not used up. 10. mol simply adds the amounts of the two reactants. 3.0 mol uses a 1:1 ratio between Cl2 and NaCl instead of 1:2.
Category: Reaction Yields
Question 18
A reaction has a theoretical yield of 50.0 g of product, and 40.0 g is actually collected. What is the percent yield?
125%
80.0%
✓
40.0%
20.0%
Correct Answer
80.0%
Percent yield is actual yield divided by theoretical yield times 100%: 40.0 g / 50.0 g x 100% = 80.0%. 125% divides the theoretical yield by the actual yield. 40.0% reports the actual mass as if it were a percentage. 20.0% is the percentage of product that was not recovered.
Category: Reaction Yields
Question 19
A 25.0 mL sample of HCl is exactly neutralized by 20.0 mL of 0.100 M NaOH. The acid and base react in a 1:1 ratio. What is the concentration of the HCl?
0.125 M
0.100 M
0.0800 M
✓
0.00200 M
Correct Answer
0.0800 M
Moles of NaOH = 0.0200 L x 0.100 M = 0.00200 mol, which equals the moles of HCl, so the HCl concentration is 0.00200 mol / 0.0250 L = 0.0800 M. 0.125 M swaps the two volumes. 0.100 M assumes the acid matches the base concentration even though the volumes differ. 0.00200 M is the number of moles of HCl, not its concentration.
Category: Chemical Analysis
Question 20
In a titration, the point at which the amount of titrant added exactly matches the amount required to react completely with the analyte, according to the balanced equation, is the
End point
Saturation point
Isoelectric point
Equivalence point
✓
Correct Answer
Equivalence point
The equivalence point is defined by stoichiometry: the added titrant has exactly reacted with all of the analyte. End point is the volume actually observed, such as where an indicator changes color, which in a good method lies very close to the equivalence point but is a separate idea. Saturation point describes a solution holding the maximum dissolved solute. Isoelectric point is a term from protein and amino acid chemistry.
Category: Chemical Analysis