Regents Chemistry Practice Problems : FORMULAS, EQUATIONS AND CHEMICAL REACTIONS

Calcium hydride is an ionic compound because it is composed of a metal and a nonmetal. 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. Ca²⁺ H^-1 = Ca₁H₂ = CaH₂

Answer: 2

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

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

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.

CuS is an ionic compound because it has a metal and a nonmetal. To name an ionic compound with a metal and a nonmetal, we write 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 neutral.

Answer: 3

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

In a single replacement reaction, one element replaces another: A +BC –> B +AC

Choice 1 is synthesis. Choice 3 is decomposition. choice 4 is double replacement.

Answer: 2

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