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Ch.4 - Chemical Reactions and Chemical Quantities
Chapter 4, Problem 34c

Balance the equation and calculate how many moles of N2 form when each quantity of reactant completely reacts. N2H4(l) → NH3(g) + N2(g) c. 33.9 g N2H4

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Step 1: Write the unbalanced chemical equation: \( \text{N}_2\text{H}_4(l) \rightarrow \text{NH}_3(g) + \text{N}_2(g) \).
Step 2: Balance the chemical equation. Start by balancing the nitrogen atoms. Since there are 2 nitrogen atoms in \( \text{N}_2\text{H}_4 \), you need 2 \( \text{NH}_3 \) molecules to balance the nitrogen: \( \text{N}_2\text{H}_4(l) \rightarrow 2\text{NH}_3(g) + \text{N}_2(g) \).
Step 3: Check the balance of hydrogen atoms. \( \text{N}_2\text{H}_4 \) has 4 hydrogen atoms, and 2 \( \text{NH}_3 \) molecules have 6 hydrogen atoms. Adjust the equation to balance hydrogen: \( 3\text{N}_2\text{H}_4(l) \rightarrow 4\text{NH}_3(g) + 3\text{N}_2(g) \).
Step 4: Calculate the molar mass of \( \text{N}_2\text{H}_4 \) using the atomic masses: \( 2 \times 14.01 + 4 \times 1.01 = 32.05 \text{ g/mol} \).
Step 5: Convert 33.9 g of \( \text{N}_2\text{H}_4 \) to moles using its molar mass: \( \text{moles of } \text{N}_2\text{H}_4 = \frac{33.9 \text{ g}}{32.05 \text{ g/mol}} \). Use the balanced equation to find moles of \( \text{N}_2 \) produced: \( \text{moles of } \text{N}_2 = \frac{3}{3} \times \text{moles of } \text{N}_2\text{H}_4 \).

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Key Concepts

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

Balancing Chemical Equations

Balancing chemical equations involves ensuring that the number of atoms of each element is the same on both sides of the equation. This is crucial for obeying the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. In the given reaction, balancing will help determine the stoichiometric relationships between the reactants and products.
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Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on balanced equations. It allows chemists to predict how much of a product will form from a given amount of reactant. In this case, stoichiometry will be used to calculate the moles of N2 produced from the given mass of N2H4, using the molar ratios derived from the balanced equation.
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Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is essential for converting between grams and moles in stoichiometric calculations. For N2H4, the molar mass must be calculated to convert the given mass (33.9 g) into moles, which will then be used to find the amount of N2 produced in the reaction.
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Hydrobromic acid dissolves solid iron according to the reaction:

Fe(s) + 2 HBr(aq) → FeBr2(aq) + H2(g)

What mass of HBr (in g) do you need to dissolve a 2.80-g pure iron bar on a padlock? What mass of H2 would the complete reaction of the iron bar produce?