High School Chemistry : Help with Molecular Weight and Molar Mass

Study concepts, example questions & explanations for High School Chemistry

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Example Questions

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Example Question #11 : Stoichiometry

How many protons does potassium-40 have?

Possible Answers:

Correct answer:

Explanation:

The number of protons is equal to the atomic number of an element. The atomic number of an element can usually be found on the top right of the element box in the periodic table. The chemical symbol for potassium is K. Using all this information the atomic number of  is 19, therefore the number of protons in potassium-40 is 19.

Example Question #11 : Stoichiometry

What is the chemical symbol for an atom with  and ?

Possible Answers:

Correct answer:

Explanation:

To determine the chemical symbol of an atom we can use the atomic number because no two elements have the same atomic number. The atomic number is equal to the number of protons, and is usually located on the top left of the element's box in the periodic table. This atom has 8 protons, therefore the atomic number is 8. Using this information it can be determined that the chemical symbol for an atom with 8 protons is O, oxygen.

Example Question #11 : Stoichiometry

Elements in the periodic table are arranged according to their __________.

Possible Answers:

atomic mass

relative size

relative activity

atomic number

Correct answer:

atomic number

Explanation:

The elements are arranged based on their atomic number. The atomic number can usually be found on the top left of each element's box. The atomic number increased from left to right as you move across the periodic table. 

Example Question #14 : Stoichiometry

What is the mass in grams of  of magnesium?

Possible Answers:

Correct answer:

Explanation:

To determine the mass in grams of  of magnesium, we need to know the molecular mass of magnesium, which can be found on the periodic table.

The molecular mass of magnesium is  . Now we have all the information we need to convert moles to grams:

In the above conversion, the moles cancel to leave the answer in grams. 

Example Question #11 : Stoichiometry

How many moles of potassium are there in  of potassium?

Possible Answers:

Correct answer:

Explanation:

To determine the number of moles in  of potassium, we need to convert grams to moles. To convert grams to moles we need to determine the atomic mass of potassium using the periodic table.  The atomic mass of potassium is

Using this information we can convert grams to moles: 

 

Example Question #16 : Stoichiometry

How many moles of tungsten are in  atoms?

Possible Answers:

Correct answer:

Explanation:

To convert atoms to moles we need to know that there are  atoms in one mole. 

Example Question #17 : Stoichiometry

How many atoms are in  of ?

Possible Answers:

Correct answer:

Explanation:

To determine the how many atoms are in  of aluminum, we need to also know that there are  atoms in one mole.

Example Question #18 : Stoichiometry

What is the mass in grams of  of ?

Possible Answers:

Correct answer:

Explanation:

To determine the mass in grams of of nickel, we first need to convert atoms to moles. To do this we need to know that . Then we will convert moles to grams using the atomic mass of nickel, which can be found on the periodic table. The atomic mass of  is

Now we have all of the information we need to perform the conversion:

Example Question #11 : Help With Molecular Weight And Molar Mass

How many atoms of sulfur are in  of ?

Possible Answers:

Correct answer:

Explanation:

The conversion of grams to atoms is a two-step process. First, we need to convert grams to moles using the molecular mass of sulfur. The molecular mass of sulfur can be found on the periodic table and is equal to 

Then we will convert moles to atoms using our knowledge that . Using all this information we will perform the following conversion:

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