GRE Subject Test: Chemistry : General Chemistry

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #1 : Reaction Chemistry

When 15.5 grams of  is used in the given reaction, how many moles of  is produced?

Possible Answers:

Correct answer:

Explanation:

We must first calculate the molecular weight of  to use as a conversion factor:

For every mole of  reacted, one mole of  is produced.

Therefore, 0.1092 moles of  is produced.

Example Question #2 : Reaction Chemistry

When 20.0 grams of  is used in the given reaction, how many moles of  is produced?

Possible Answers:

Correct answer:

Explanation:

We must first calculate the molecular weight of  to use as a conversion factor:

For every 1 mole of  reacted, 2 moles of  is produced.

Therefore,the number of moles of  produced will be double the number of moles of  used up in the reaction:

Example Question #1 : Chemical Equations

What is the smallest whole-number coefficient that can be designated to the water molecule that would balance the chemical equation given?

Possible Answers:

Correct answer:

Explanation:

The number of atoms on the reactant side of the chemical equation are:

The number of atoms on the product side of the chemical equation are:

In a balanced chemical equation, the total number of atoms of each element must be equal on the reactant and product side of the chemical equation. There is one more oxygen and two more hydrogen atoms on the reactant side of the chemical equation. Giving the water molecule on the product side of the chemical equation would balance the chemical equation so that both sides contain six oxygens and five hydrogens:

Example Question #141 : General Chemistry

What is the smallest whole number coefficient that can be designated to the  molecule that would balance the chemical equation given?

Possible Answers:

Correct answer:

Explanation:

The process of balancing a chemical equation is called "balancing by inspection" which is done by trial and error. The easiest way to approach the problem given is by balancing the species containing an element with the most complicated formula occurring in one reactant and one product. In this case, we can either begin by balancing the carbon or hydrogen in the propane molecule. Let's start with the carbon by adding a coefficient of 3 to the carbon dioxide molecule on the product side of the equation which gives:

Let's now balance the hydrogens by adding a coefficient of 4 to the product side of the equation:

Lastly, we can balance the number of oxygens in the reaction:

 

Example Question #10 : Chemical Equations

If  of  is used for the reaction given, how many grams of  was formed?

Possible Answers:

Correct answer:

Explanation:

Based on the molecular equation, for every 1 mole of  reacted, 1 mole of  is formed as a product. Using dimensional analysis, we can use this relationship to determine the moles of  reacted:

We must calculate the molecular weight of :

To calculate the number of grams of , we need to convert the number of moles of  to grams using the molecular weight as a conversion factor.

Example Question #11 : Reaction Chemistry

If  of  is used for the reaction given, how many grams of  was formed?

Possible Answers:

Correct answer:

Explanation:

Based on the molecular equation, for every 4 moles of  reacted, 2 moles of  is formed as a product:

We must calculate the molecular weight of :

To calculate the number of grams of , we need to convert the number of moles of  to grams using the molecular weight as a conversion factor.

Example Question #12 : Reaction Chemistry

If  of  is formed during the reaction given, how many grams of  was used?

Possible Answers:

Correct answer:

Explanation:

Based on the molecular equation, for every 2 moles of  formed, 1 mole of  is reacted:

We must calculate the molecular weight of :

To calculate the number of grams of , we need to convert the number of moles of  to grams using the molecular weight as a conversion factor.

Example Question #13 : Reaction Chemistry

If  of  is formed during the reaction given, how many grams of  was used?

Possible Answers:

Correct answer:

Explanation:

Based on the molecular equation, for every 3 moles of  formed, 1 mole of  is reacted:

We must calculate the molecular weight of :

To calculate the number of grams of , we need to convert the number of moles of  to grams using the molecular weight as a conversion factor.

Example Question #1 : Oxidation Reduction Reactions

Based on the chemical equation given, which of the following compounds underwent reduction?

Possible Answers:

Correct answer:

Explanation:

The type of chemical reaction given is called an oxidation-reduction reaction. In this type of reaction, there is a transfer of electrons from one element to another. The species gaining an electron is said to be reduced and the species losing an electron is oxidized. Manganese ion goes from a  to a  oxidation state. Therefore, manganese is reduced in the chemical reaction because it gains electrons from the compound being oxidized (iron).

Example Question #1 : Oxidation Reduction Reactions

Methane combusts in the presence of oxygen according to the following reaction:

Which of the following statements is true concerning the reaction?

Possible Answers:

Oxygen has a charge of  throughout the entire reaction

Oxygen is oxidized in the reaction

Carbon has an initial oxidation state of

Carbon is oxidized in the reaction

will be the limiting reagent

Correct answer:

Carbon is oxidized in the reaction

Explanation:

By comparing the oxidation number of an atom as a reactant and its oxidation number as a product, we can determine if the atom has been oxidized or reduced. In increase in oxidation number indicates a loss of electrons, or oxidation. A decrease in oxidation number signals a gain of electrons, or reduction.

For electrochemistry, you should familiarize yourself with the traditional oxidation states of hydrogen , halogens , oxygen , and elemental atoms

Carbon is initially in the form of methane, meaning that it is attached to four hydrogen atoms. The molecule is neutral, and each hydrogen has an oxidation number of . Carbon must have an initial oxidation state of in order to balance the molecular charge.

In the a product, carbon is attached to two oxygens, each with a charge of . Again, the molecule is neutral, so carbon must balance these charges. This means that carbon's final oxidation state is .

Since carbon went from an oxidation state of  to , we can conclude that carbon has been oxidized in the reaction.

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