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AP Chemistry Chemistry — practice by topic

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Chemistry topics

Composition of Substances and Solutions (23)Equilibria of Other Reaction Classes (23)Solutions and Colloids (23)Stoichiometry of Chemical Reactions (23)Thermodynamics (23)Acid-Base Equilibria (22)Atoms, Molecules, and Ions (22)Chemical Bonding and Molecular Geometry (22)Electronic Structure and Periodic Properties (22)Gases (22)Thermochemistry (22)Intermolecular Forces, Liquids and Solids (21)Electrochemistry (20)Fundamental Equilibrium Concepts (20)Electrochemical Series (15)Kinetics (15)

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To count atoms easily, how can the formula of calcium phosphate, Ca3(PO4)2, be rewritten in a simpler format?Read the answer and worked solution →Calcium phosphate, Ca3(PO4)2, has a formula mass of 310.18 amu. What is the mass of 0.500 mol of calcium phosphate?Read the answer and worked solution →What is the mass of 0.443 mol of hydrazine, N2H4? Use N = 14.007 and H = 1.008 g/mol.Read the answer and worked solution →A prospector collects 15.00 g of pure gold. Given that 2.561 mol of gold has a mass of 504.4 g, how many Au atoms are in the sample?Read the answer and worked solution →

Worked practice questions

To count atoms easily, how can the formula of calcium phosphate, Ca3(PO4)2, be rewritten in a simpler format?

  1. Ca3PO4
  2. Ca3P2O8
  3. Ca3P2O12
  4. Ca2P3O8
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Correct answer: B. Ca3P2O8

1. The bracket subscript 2 multiplies everything inside the parentheses. 2. Two phosphate groups give 2 phosphorus atoms and 2 x 4 = 8 oxygen atoms. 3. With the 3 calcium atoms the simple format is Ca3P2O8. 4. Reading the bracket contents only once gives Ca3PO4, which loses a whole phosphate group. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.1 Formula Mass and the Mole Concept_

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Calcium phosphate, Ca3(PO4)2, has a formula mass of 310.18 amu. What is the mass of 0.500 mol of calcium phosphate?

  1. 31.02 g
  2. 310.2 g
  3. 620.4 g
  4. 155.1 g
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Correct answer: D. 155.1 g

1. A formula mass of 310.18 amu means a molar mass of 310.18 g/mol, since the two are numerically equal. 2. Mass equals moles multiplied by molar mass. 3. Substituting gives 0.500 mol x 310.18 g/mol = 155.1 g. 4. Units check: mol x g/mol leaves grams. 5. The value 310.2 g ignores that only half a mole is present, and 620.4 g divides by 0.500 instead of multiplying. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.1 Formula Mass and the Mole Concept_

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What is the mass of 0.443 mol of hydrazine, N2H4? Use N = 14.007 and H = 1.008 g/mol.

  1. 14.2 g
  2. 12.4 g
  3. 1.42 g
  4. 0.0138 g
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Correct answer: A. 14.2 g

1. Build the molar mass first: nitrogen contributes 2 x 14.007 = 28.014 g/mol. 2. Hydrogen contributes 4 x 1.008 = 4.032 g/mol, so the molar mass is 32.05 g/mol. 3. Mass equals moles times molar mass: 0.443 mol x 32.05 g/mol = 14.2 g. 4. The value 12.4 g uses only the nitrogen part of the molar mass, dropping the four hydrogen atoms. 5. The value 0.0138 g divides the moles by the molar mass instead of multiplying. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.1 Formula Mass and the Mole Concept_

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