AP Chemistry : Compounds

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

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Example Question #1 : Ap Chemistry

What is the percent by mass of bismuth in the compound ?

Possible Answers:

Correct answer:

Explanation:

The mass percentage of bismuth in the compound will be equal to the mass of bismuth in one mole of compound divided by the total molar mass of the compound.

Bismuth has a molar mass of . One mole of the compound would result in two moles of bismuth, a total of 417.96g.

Tellurium has a molar mass of . One mole of the compound would result in three moles of tellurium, a total of 382.8g.

Add the mass of bismuth and the mass of tellurium per mole to find the total molar mass.

Divide the mass of bismuth by the total molecular mass to find the percent by mass of bismuth in the compound.

Example Question #2 : Mass

What is the percent by mass of carbon in acetic acid ?

Possible Answers:

Correct answer:

Explanation:

The total molar mass of acetic acid is .

Carbon contributes  total. Therefore, in a sample of 1 mole of acetic acid, there are  of carbon. To find the percent by mass, we must divide the mass of carbon by the total molar mass of the compound.

Example Question #3 : Mass

Calculate the percent by mass of each element in .

Possible Answers:

Correct answer:

Explanation:

The total molar mass of lead (II) sulfate is . Lead contributes , sulfur contributes , and oxygen contributes .

The percent by mass of each element in the compound is found by dividing the mass contribution of that element by the total molar mass of the compound.

Example Question #1 : Mass

What percent by mass of  is chlorine?

Possible Answers:

Correct answer:

Explanation:

The molar mass of calcium chloride is . Chlorine has two moles per one mole of . Therefore, chloride contributes 70.9g to the total molecular mass.

To find the percent by mass of chlorine in calcium chloride, divide the contribution of chlorine by the total molar mass.

Example Question #1 : Ap Chemistry

Calculate the percent by mass of each element in .

Possible Answers:

Correct answer:

Explanation:

The total mass of one mole of aluminum (II) chromate is calculated by:

Aluminum, chromate, and oxygen contribute  and respectively. Therefore, we can divide each contribution by the total molecular mass to determine percentages by mass.

Example Question #2 : Mass

What is the percent by mass of hydrogen in ?

Possible Answers:

Correct answer:

Explanation:

The total mass of  is  and calculated by:

You can then divide hydrogen's contribution by the total molecular mass in order to find the percent mass.

Example Question #1 : Mass

What percent of the overall mass is represented by the cation(s) in ?

Possible Answers:

Correct answer:

Explanation:

The total molar mass of  is . Cesium has a +1 charge when the compound is dissolved in solution and is, thus, the cation.

Cesium contributes 265.8g to the total molecular mass.

We can find the percent by mass of cesium by dividing the mass of cesium by the total molecular mass.

Example Question #1 : Nomenclature And Functional Groups

What is the oxidation state of manganese () in the polyatomic permanganate anion ()?

Possible Answers:

Correct answer:

Explanation:

When assigning oxidation states to elements of a given compound, non-transition metal elements are assigned specific oxidation states corresponding to their group number and valence relative to a complete octet.

Group 1 elements have 1 valence electron and an oxidation state of +1.

Group 2 elements have 2 valence electrons and an oxidation state of +2. 

Group 8 elements (the noble gases) have a complete octet, thus their assigned oxidation state is 0.

Group 7 elements (halogens) have 7 valence electrons and an oxidation state of -1.

Group 6 elements such as oxygen have 6 valence electrons and an oxidation state of -2.

Permanganate has 4 oxygen atoms and an overall charge of -1. The oxidation state of the  atom may be found by first calculating the combined oxidation state of the oxygen atoms:

 

and finding the difference between their combined oxidation state and the overall charge of the ion (-1):

Example Question #3 : Nomenclature And Functional Groups

According to VSEPR theory, for the ammonium ion (), the electron-pair geometry is __________ and the molecular geometry is __________.

Possible Answers:

trigonal pyramidal . . . tetrahedral

tetrahedral . . . tetrahedral

trigonal pyramidal . . . trigonal pyramidal

tetrahedral . . . trigonal pyramidal

Correct answer:

tetrahedral . . . tetrahedral

Explanation:

Both the electron-pair and molecular geometries of ammonium are tetrahedral, as opposed to ammonia (), in which the electron-pair geometry is tetrahedral, but the molecular geometry is trigonal pyramidal. As evidenced below:

Ammonia:

Screen shot 2015 06 22 at 5.12.58 pm

Note that the difference in geometry arises in the presence of the lone pair on nitrogen in ammonia. The lone pair only contributes to molecular geometry, but does not contribute to electron-pair geometry. Bonding electrons, however, contribute to both electron and molecular geometries.

Ammonium:

Screen shot 2015 06 22 at 5.15.59 pm

In ammonium, the lone pair seen in ammonia is shared in a bond with an additional hydrogen atom. 

Example Question #8 : Nomenclature And Functional Groups

What are the mass percents of C, H, and O, respectively, in a molecule of glucose monomer ()?

Possible Answers:

Correct answer:

Explanation:

On average, the molar masses of carbon, hydrogen, and oxygen are approximately ,  , and , respectively.

The molar mass of a molecule of glucose is:

 

Each term added in the previous step is the mass of each element present in a mole of glucose, so the mass percent of its components are found by dividing their respective contributions to the molar mass by the molar mass:

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