Correct answer: 2 NaN_3(s) →  2 Na_(s) +  3 N_2(g)

Why?:

The Law of Conservation of Mass states that the total mass of the reactants is equal to the total mass of the products. This also means that chemical equations should be balanced. In balancing an equation, the number of atoms appearing on the reactant side must equal the number of atoms appearing on the product side. If there is a total of three sodium atoms on the reactant side, then there should also be a total of three sodium atoms on the product side of the equation.

Answering this question:

On the reactant side, there is 1 sodium atom and 3 nitrogen atoms. On the product side, there is 1 sodium atom and 2 nitrogen atoms. To balance the nitrogen atoms, we must look for a factor that is a multiple of 2 and 3. Since 6 is the lowest multiple of the numbers 2 and 3, we will use 6. On both sides of the reaction, there should be 6 nitrogen atoms. So we write a coefficient of 2 for NaN₃ and 3 for N₂. This way, the number of nitrogen atoms on both sides of the reaction is equal. However, we are still left with the sodium atom to balance. Since there are now 2 atoms of sodium on the reactant side and only 1 on the product side, then we should write a coefficient of 2 for Na on the product side. With this, the number atoms on the reactant side are balanced with the number of atoms on the product side.

So the correct answer is 2 NaN_3(s) → 2 Na_(s) + 3 N_2(g)

Correct answer: 2734 kJ

Why?:

This chemical reaction shows the combustion of one mole of ethanol producing 1367 kJ. This reaction is exothermic as heat is released. The amount of heat released by one mole of ethanol is 1367 kJ. When the amount of ethanol undergoing combustion is increased, the amount of heat released also increases.

Answering this question:

The amount of heat released by a combustion process is proportional to the amount of reactant consumed. Simply multiply the amount of heat released with the increase in the amount of the reagent.

So the amount of heat that will be produced will be 2734 kJ.

Correct answer: 781 g Na₃PO₄

Why?:

The mass of a compound is the sum total of the gram-atomic masses of all the atoms in the compound. It can easily be calculated using the gram-formula mass of a given formula. If the number of moles of a compound is given, a dimensional analysis can be done to obtain the mass of the compound.

In the rules of significant figures, the product in a multiplication process follows the entry with the least number of significant figures. In this case, 4.74 and 164 both have three significant figures. So the product must contain three significant figures as well. Therefore, 780. 64 is rounded off to 781.

So the best answer is 781 g Na₃PO₄.

Correct answer: 6 oxygen atoms

Why?:

The number of atoms in a compound or molecule can be calculated by multiplying the subscript with the coefficient. In this regard, the absence of a subscript or coefficient means the presence of the number 1. In molecular formulas a subscript indicates the multiplicity of a single element if it immediately follows an element's symbol. If a subscript immediately follows parentheses then it indicates the multiplicity of the larger group inside the parentheses.

Answering this question:

The compound produced is Fe(NO₃)₂. Since a subscript of 2 immediately follows a parenthesis, this indicates that there are 2 NO₃ ions. In a single NO₃ ion, there are 3 oxygen atoms as indicated by the subscript 3 that immediately follows the symbol for oxygen. Simply multiply the subscript of oxygen in NO₃ by how many NO₃ ions there are. In this case, it is (3)(2) = 6.

So there are 6 oxygen atoms.

Correct answer: (3) CH₃CH₂OH

Why?:

In any balanced chemical equation, the masses of the substances involved are conserved. This is in accordance with the Law of Conservation of Mass. This also means that the number of moles of an atom in the reactant side is equal to the number of moles of a corresponding atom on the product side. The reactant, glucose, contains 6 moles of carbon, 12 moles of hydrogen, and 6 moles of oxygen. These exact numbers of moles of each of the atoms making up glucose must appear on the product side

Answering this question:

The product side of the equation already contains 2 moles of CO₂. It has a total of 2 moles of carbon and 4 moles of oxygen. Because of the coefficient 2 that is already present, the unknown species must contain 2 moles of carbon, 6 moles of hydrogen and 1 mole of oxygen. These exact numbers of moles of atoms on the product side will balance the equation. Among all the choices, it is choice (3) that contains 2 moles of carbon, 6 moles of hydrogen and 1 mole of oxygen.

So (3) is the correct answer.

Correct answer: (3) HClO₃ (gram-formula mass = 84 g/mol)

Why?:

Percent composition if the relative amounts of atoms in a compound or molecule. To obtain the percent composition of an atom in a molecule, simply divide the gram-atomic mass of the atom by the gram-formula mass of the molecule. Multiply this by 100 to get the percent value. Different units of mass can be used as long as the same kind of unit is used for the atom and for the molecule.

Answering this question:

The gram-atomic mass of chlorine is 35.45 g/mol. Simply divide this with the gram-formula mass given in the choices. Multiply with 100 to get the percent value.

In (1), (35.45/52)(100) = 68%. In (2), (35.45/68)(100) = 52%. In (3), (35.45/84)(100) = 42%. You can even stop here if you are sure of your math. But to be so sure, you can continue with (4). In (4), (35.45/100)(100) = 35%.

So the correct answer is (3).

Correct answer: (3) C₄H₈

Why?:

The empirical formula shows the relative number of atoms in a molecule. This may or may not be exactly the same as the molecular formula. The molecular formula shows the ratio of atoms a molecule actually contains. The molecular formula for glucose is C₆H₁₂O₆. Its empirical formula could be C₂H₄O₂. The subscripts in an empirical formula are small whole number ratio of the subscripts of the molecular formula. In other words, the subscripts of the atoms in the molecular formula are divisible by the subscripts of the corresponding atoms in the empirical formula by a single factor.

Given a molar mass and an empirical formula, the molecular formula can be easily obtained. Simply obtain the empirical formula mass. Divide the molar mass by the empirical formula mass. The number obtained is multiplied to the subscripts of the atoms in the empirical formula. The resulting formula is the molecular formula.

Answering this question:

Obtain first the empirical formula mass. For CH₂, it is 12 for carbon, and 2 for the 2 atoms of hydrogen. Thus, the empirical formula mass is 14 grams per mole. Since the given molar mass is 56 grams per mole, (1) can be eliminated. Divide 56 by 14. This will give a value of 4. Multiply this with the subscripts of the atoms in the empirical formula. For carbon, it is 1(4) = 4. For hydrogen, it is 2(4)=8. So the molecular formula should be C₄H₈.

So the correct answer is (4).

Correct answer: (2) 1:2

Why?:

In a balanced chemical equation, the number of moles of each of the species is represented by the coefficients. The coefficients are whole numbers written to the left of the symbol for the species. With species, we mean ions, atoms, compounds of molecules. However, when the coefficient is just 1, there is no need to write it anymore. So if there is no coefficient to the left of a species, then the coefficient is 1. In this balanced equation, the coefficient of F₂ is 1. The coefficient of H₂ is also 1. And the coefficient of HF is 2. So the mole ratio of H₂ to HF is 1:2.

Answering this question:

The question is asking for the mole ratio of H₂ to HF. Just simply look at the coefficients of each of the species. Whatever are the coefficients of the species involved, those are the numbers indicated in the ratio. The first number in the ratio is always that of the species first mentioned. In this case, it is H₂. The second number is that of the species mentioned next. In this case, it is HF.

So the answer is (2).