Molar Mass

How many of each type of atom are in Ca(OH)2?

​ (a)   Ca

​ (b)   O

2 ​ (c)  

​ (d)   atoms total

• 1
• 2
• H
• 5
• 3
• OH
• Ca

Answer explanation

There is no number after the Ca, so there is one calcium. There is one O inside the parentheses, and a 2 outside the parentheses, so you multiply 1 x 2 = 2 oxygen. The same applies to the hydrogen.

1 Ca + 2 O + 2 H = 5 atoms total

Tags

NGSS.MS-PS1-1

How many of each type of atom are in Na2SO4?

​ (a)   Na

​ (b)   S

​ (c)   O

​ (d)   atoms total

• 2
• 1
• 4
• 7
• 3
• 5
• 6

Answer explanation

There is a 2 after the Na, so there are 2 Na. There is no number after the S, so there is 1 S. There is a 4 after the O, so there are 4 O.

Tags

NGSS.MS-PS1-1

What is the molar mass of carbon, rounded to the nearest whole number?

• 6 grams

• 10 grams

• 11 grams

• 12 grams

• 13 grams

Answer explanation

The atomic mass of carbon is 12.011 (your periodic table might give a slightly different number). Look at the number immediately after the decimal. If it is a 0-4, the numbers in front of the decimal stay the same. So 12.011 rounds to 12.

If you picked 6, that is the atomic number, not the atomic mass.

What is the molar mass of oxygen, rounded to the nearest whole number?

• 8 grams

• 15 grams

• 16 grams

• 17 grams

Answer explanation

The atomic mass of oxygen is 15.999 (your periodic table might give a slightly different number). Look at the number immediately after the decimal. If it is a 5-9, the numbers in front of the decimal goes up by one. So 15.999 rounds to 16.

If you picked 8, that is the atomic number, not the atomic mass.

Write the atomic mass of helium, rounded to the nearest whole number. Just type in the number, not the unit. Helium = _ g.

Answer explanation

The atomic mass of helium rounds to 4, so the molar mass of helium is 4.

What is the molar mass of H2, rounded to the nearest whole number?

• 1 gram

• 2 grams

• 3 grams

• 4 grams

Answer explanation

The molar mass of hydrogen is 1. You were asked to find the mass of H2, so you have to multiply the molar mass of H and then multiply by the number of hydrogen atoms.

1 x 2 = 2 grams.

What is the molar mass of H2O, rounded to the nearest whole number?

• 3 grams

• 12 grams

• 17 grams

• 18 grams

Answer explanation

The molar mass of hydrogen is 1. The molar mass of oxygen is 16. You were asked to find the mass of H2O, so you have to multiply the molar mass of H and then multiply by the number of hydrogen atoms. Then add the molar mass of oxygen.

1 x 2 = 2 grams.

16 x 1 = 16 grams.

2 + 16 = 18 grams.

What is the molar mass of Al2O3, rounded to the nearest whole number?

• 48 grams

• 54 grams

• 102 grams

• 43 grams

Answer explanation

The molar mass of aluminum is 27. The molar mass of oxygen is 16. You were asked to find the mass of Al2O3, so you have to multiply the molar mass of aluminum and then multiply by the number of aluminum atoms. Repeat for oxygen, then add your totals.

What is the molar mass of Ca(OH)2, rounded to the nearest whole number?

• 114 g

• 40 g

• 57 g

• 74 g

Answer explanation

Ca(OH)2 contains 1 calcium atom, 2 oxygen atoms, and 2 hydrogen atoms. Find the mass of each atom and multiply it by the number of that type of atom in the formula.

What is the molar mass of Na2SO4?

• 112 g

• 122 g

• 61 g

• 142 g

Answer explanation

Find the mass of sodium (23) and multiply it by the number of sodiums 23 x 2 = 46. Find the mass of sulfur (32) and multiply it by the number of sulfurs 32 x 1 = 32. Find the mass of oxygen (16) and multiply it by the number of oxygens 16 x 4 = 64. Add your totals together for the final answer.