June 2023
36 Which formula represents calcium hydride? (1) CaH
(2) CaH2
(3) CaOH
(4) Ca(OH)2
Solution: Calcium hydride is an ionic compound because it is composed of a metal and a nonmetal. Calcium is a metal because it is to the left of the staircase on the Periodic Table. Hydrogen is a nonmetal (it is also on the left) but it is an exception. To write the formula, we can look at the oxidation number of each element on the Periodic Table. Calcium is +2 and hydrogen is -1. We then cross these numbers so they become subscripts of the partners.
Ca^2+ H^1-
Ca1H2 = CaH2
Answer: 2
38 Given the equation representing a reaction: F2g) + 2KCl(aq) → 2KF(aq) + Cl2g) Which type of chemical reaction is represented by the equation? (1) synthesis (2) decomposition (3) single replacement
(4) double replacement
Solution: In synthesis two or more molecules come together to form one bigger molecule: A+B--> AB
Decomposition is the opposite of synthesis: AB --> A+B
In a single replacement reaction, one element replaces another: A +BC --> B +AC
In a double replacement reaction partners get exchanged: AB +CD --> AD +CB
Answer: 3
January 2023
35 Which formula represents chromium(III) oxide?
(1) CrO3
(2) Cr3O
(3) Cr2O3
(4) Cr3O2
Solution: Chromium oxide is an ionic compound because it is composed of a metal and a nonmetal. The charge of Cr is 3+ according to the roman numeral in the formula. The charge of O is -2 according to the Periodic Table. We cross multiply the numbers get the correct formula. Cr should get 2 from oxygen and O should get 3 from the chromium. Answer: We get Cr2O3, choice 3
38 Given the equation representing a reaction: 2NaCl → 2Na + Cl2 Which type of reaction does this equation represent?
(1) double replacement
(2) decomposition
(3) synthesis
(4) single replacement
Solution: Here are some easy ways to recognize each reaction.
Double replacement is AB + CD --> AD + CB,
Decomposition is AB --> A + B
Synthesis is A+B --> AB
Single Replacement is A +BC --> AC + B
Answer: The reaction represented above is decomposition, choice 2.
August 2022
35 What is a chemical name of the compound CuS?
(1) copper(I) sulfide
(2) copper(I) sulfate
(3) copper(II) sulfide
(4) copper(II) sulfate
Solution: CuS is an ionic compound because it has a metal with a nonmetal. To name an ionic compound with a metal and a nonmetal, we say the name of the metal followed by nonmetal ending in "ide". This eliminates choices 2 and 4 since they end in "ate" (that would be polyatomic ion).
Copper is also a transition metal and we need to use Roman numerals when there is a transition metal. For ionic compounds, we exchange their oxidation numbers (charges) to arrive at the formula. Sulfur is group 16 on the Periodic Table with an oxidation number of -2 (top right number). The formula is CuS which means one to one ratio of the atoms. If sulfur is -2 then copper has to be +2 to balance sulfur out and make it 1 to 1 ratio.
Answer: 3
January 2020
36 What is the chemical formula for sodium oxalate?
(1) NaO (2) Na2O
(3) NaC2O4
(4) Na2C2O4
Solution: Sodium oxalate is an ionic compound because it is composed of a metal (sodium) and a polyatomic ion (oxalate). Table E lists polyatomic ions. Oxalate is C2O4^2–. The oxidation number (charge) of sodium ion can be found on the Periodic Table and is +1. The charge numbers get exchanged to make the correct ionic formula.
Na^+1 C2O4^2–
Na2(C2O4)1 = Na2C2O4
Answer: 4
39 Which equation represents a single replacement reaction?
(1) 2Al(s) + 3Cl2(g) → 2AlCl3(s)
(2) 2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g)
(3) 2AlCl3(s) → 2Al(s) + 3Cl2(g)
(4) AlCl3(aq) + 3KOH(aq) → Al(OH)3(s) + 3KCl(aq)
Solution: In a single replacement reaction, one element replaces another: A +BC --> B +AC
Answer: 2
August 2019
34 What is the chemical name of the compound NH4SCN? (1) ammonium thiocyanate
(2) ammonium cyanide (3) nitrogen hydrogen cyanide
(4) nitrogen hydrogen sulfate
Solution:NH4SCN is made up from two polyatomic ions. Table E lists polyatomic ions with their formulas and names. NH4+ is ammonium and SCN- is thiocyanate. The name of an ionic compound consisting of two polyatomic ion is written as follows: name of the first polyatomic ion + name of the second polyatomic ion.
Answer: 1
January 2019
33 Given the equation representing a chemical reaction: NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s) This reaction is classified as a (1) synthesis reaction (2) decomposition reaction (3) single replacement reaction
(4) double replacement reaction
Solution:In synthesis two or more molecules come together to form one bigger molecule: A+B--> AB
Decomposition is the opposite of synthesis: AB --> A+B
In a single replacement reaction, one element replaces another: A +BC --> B +AC
In a double replacement reaction partners get exchanged: AB +CD --> AD +CB
Answer: 4
34 What is the formula for iron(II) oxide?
(1) FeO
(2) FeO2 (3) Fe2O
(4) Fe2O3
Solution: This compound consists of a metal (iron) with a nonmetal (oxygen). The roman number represents the oxidation number (charge) of iron. The oxidation number of oxygen is on the Periodic Table and is -2.
Each element gives the oxidation number to its' partner for the correct formula.
Fe^2+ O^2-
Fe2O2 = Fe1O1 or FeO
Answer: 1
August 2018
34 Which formula represents ammonium nitrate? (1) NH4NO3
(2) NH4NO2 (3) NH4(NO3)2
(4) NH4(NO2)2
Solution: ammonium nitrate is made up of two polyatomic ions. Table E lists polyatomic ions with their formulas and names. Ammonium is NH4+ and nitrate is NO3-. +1 and -1 cancel to give us a neutral molecule.
Answer: 1
37 Given the balanced equation representing a reaction: 2H2 + O2 → 2H2O + energy Which type of reaction is represented by this equation? (1) decomposition (2) double replacement
(3) single replacement
(4) synthesis
Solution: In synthesis two or more molecules come together to form one bigger molecule: A+B--> AB
Decomposition is the opposite of synthesis: AB --> A+B
In a single replacement reaction, one element replaces another: A +BC --> B +AC
In a double replacement reaction partners get exchanged: AB +CD --> AD +CB
Answer: 4
June 2018
38 Given the equation representing a reaction: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) Which type of chemical reaction is represented by the equation? (1) synthesis (2) decomposition (3) single replacement
(4) double replacement
Solution: In synthesis two or more molecules come together to form one bigger molecule: A+B--> AB
Decomposition is the opposite of synthesis: AB --> A+B
In a single replacement reaction, one element replaces another: A +BC --> B +AC
In a double replacement reaction partners get exchanged: AB +CD --> AD +CB
Answer: 3
June 2017
34 What is the chemical formula for sodium sulfate?
(1) Na2SO4
(2) Na2SO3 (3) NaSO4
(4) NaSO3
Solution: Sodium sulfate is an ionic compound that consists of a metal (sodium) and a polyatomic ion, sulfate. Table E on the Reference Table lists common polyatomic ions with their names, formulas and oxidation numbers. Sulfate is SO4 ^2-. The oxidation number for sodium can be found on the Periodic Table and is +1.
Putting the name together we write both parts with their oxidation numbers and then we criss cross the numbers and make them into subscripts.
Na^+1 SO4^2-
Na2SO4
Answer: 1
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