# AP Chemistry : Compounds

## Example Questions

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### Example Question #1 : Ap Chemistry

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

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 : Ap Chemistry

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

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 : Ap Chemistry

Calculate the percent by mass of each element in .

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 #4 : Ap Chemistry

What percent by mass of  is chlorine?

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 #5 : Ap Chemistry

Calculate the percent by mass of each element in .

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 #6 : Ap Chemistry

What is the percent by mass of hydrogen in ?

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 #7 : Ap Chemistry

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

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 #31 : Compounds And Molecules

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

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 #32 : Compounds And Molecules

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

tetrahedral . . . trigonal pyramidal

trigonal pyramidal . . . tetrahedral

tetrahedral . . . tetrahedral

trigonal pyramidal . . . trigonal pyramidal

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:

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:

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

### Example Question #33 : Compounds And Molecules

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