GRE Subject Test: Chemistry : Chemical Equations

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

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Example Questions

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Example Question #44 : Chemical Reactions

What is the net ionic equation for the ion exchange reaction between ferrous sulfate and calcium iodide? Assume all compounds are soluble.

Possible Answers:

None of the available answers

Correct answer:

Explanation:

First, we must know what ferrous sulfate is. Ferrous refers to , and sulfate has the formula . When we combine the two together we get .

Calcium is a divatent cation and iodide is a monovalent anion, so their salt is . The ion exchange reaction is then:

Example Question #4 : Balancing Chemical Equations

Select the net ionic equation from this molecular reaction:

Possible Answers:

None of the other choices

Correct answer:

Explanation:

The net ionic equation is derived by removing all spectator ions from the total ionic equation (in which all ions are listed). To put it another way, the net ionic equation involves only the ions that participate in a reaction which, in this case, is the precipitation of barium sulfate.

Begin by writing all aqueous compounds in their dissociated (ionic) forms.

Cancel out any ions that appear in equal quantities on both sides of the equation. In this case, we can cancel the nitrate and potassium ions.

This is our net ionic equation.

Example Question #4 : Fundamental Reaction Concepts

What is the balanced chemical equation for the combustion of butane ?

Possible Answers:

Correct answer:

Explanation:

Combustion is the chemical reaction of a hydrocarbon with molecular oxygen, and it always produces carbon dioxide and water. Knowing the reactants and products, the unbalanced equation must be: 

We start by balancing the hydrogens. Since there are 10 on the left and only 2 on the right, we put a coefficient of 5 on water.

Similarly, we balance carbons by putting a 4 on the carbon dioxide.

To find the number of oxygens on the right, we multiply the 4 coefficient by the 2 subscript on O (which gets us 8 oxygens) and then add the 5 oxygens from the 5 water molecules to get a total of 13. The needed coefficient for  on the left would then have to be 13/2.

Because fractional coefficients are not allowed, we mutiply every coefficient by 2 to find our final reaction:

Example Question #493 : Ap Chemistry

Determine whether or not solid aluminum reacts with aqueous zinc chloride. If it does, determine the balanced equation for the reaction.

Possible Answers:

No reaction occurs

Correct answer:

Explanation:

When we check the activity series, it is fairly easy to see that aluminum metal is more reactive than zinc metal. So, in this case, the two metals undergo a redox reaction, where the aqueous  is reduced to solid , and the solid  is oxidized to aqueous . These charges are the common oxidation states for zinc and aluminum and should be memorized.

Because  is the new species, it bonds with 3  ions. The unbalanced equation is:

We note that there are 2 chlorine atoms on the left and 3 chlorine atoms on the right. To balance, we use a 3 coefficient on the left and a 2 coefficient on the right. This gives a total of 6 chlorine atoms on eahc side.

However, now we have also increased the amounts of zinc and aluminum. We copy the necessary coefficients to balance those—2 for aluminum on the left, 3 for zinc on the right—and we are done:

Example Question #135 : General Chemistry

What will be the coefficient for  once the following equation is balanced?

Possible Answers:

Correct answer:

Explanation:

In the unbalanced equation from the question, the reactant side contains two hydrogen atoms and one oxygen atom. On the product side of the equation, there are four hydrogens and two oxygen atoms. So for every oxygen atom there are two hydrogen atoms giving a ratio of  (oxygen : hydrogen). Putting a coefficient of two in front of the water molecule on the reactant side of the equation balances the equation. Therefore, there are four hydrogen and two oxygen atoms on both sides of the balanced chemical equation.

Example Question #1 : Chemical Equations

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 : Chemical Equations

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 #3 : 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 #4 : Chemical Equations

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 #4 : 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.

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