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Ch.3 - Molecules, Compounds & Chemical Equations
Chapter 3, Problem 112

Balance each chemical equation. a. Na2S(aq) + Cu(NO3)2(aq) → NaNO3(aq) + CuS(s) b. N2H4(l) → NH3(g) + N2(g) c. HCl(aq) + O2(g) → H2O(l) + Cl2(g) d. FeS(s) + HCl(aq) → FeCl2(aq) + H2S(g)

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Step 1: For equation (a), start by balancing the number of sodium (Na) atoms. There are 2 Na atoms in Na2S, so ensure there are 2 Na atoms in NaNO3 by placing a coefficient of 2 in front of NaNO3.
Step 2: Next, balance the sulfur (S) atoms in equation (a). There is 1 S atom in both Na2S and CuS, so they are already balanced.
Step 3: Balance the copper (Cu) atoms in equation (a). There is 1 Cu atom in both Cu(NO3)2 and CuS, so they are balanced.
Step 4: Balance the nitrate (NO3) groups in equation (a). There are 2 NO3 groups in Cu(NO3)2, so ensure there are 2 NO3 groups in NaNO3 by placing a coefficient of 2 in front of NaNO3.
Step 5: For equation (b), balance the nitrogen (N) atoms. There are 2 N atoms in N2H4, so ensure there are 2 N atoms in the products by having 1 N2 molecule and 2 NH3 molecules.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Law of Conservation of Mass

The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction. This principle is fundamental when balancing chemical equations, as it requires that the number of atoms of each element must be the same on both sides of the equation. Understanding this law helps ensure that all reactants and products are accounted for in the balanced equation.
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Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on the balanced equation. It involves using mole ratios derived from the coefficients of the balanced equation to determine the amounts of substances involved. Mastery of stoichiometry is essential for accurately balancing equations and predicting the outcomes of chemical reactions.
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Types of Chemical Reactions

Recognizing the types of chemical reactions—such as synthesis, decomposition, single replacement, and double replacement—can aid in balancing equations. Each type has specific patterns and rules that can simplify the balancing process. Understanding these categories allows for a more strategic approach to identifying how reactants transform into products.
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