How many moles are contained in the following:
a. 5.89 g KBr
b. 5.3 kg NaCl
c. 7.9 mg KNO3
moles = grams/molar mass
Can you check my answers?
a. 0.0494948 moles
b. 90.686651 moles
c. 0.0000781 moles
Except for small differences, probably due to our not using the exact same molar mass, these are ok. I would make one point. If your prof watches the significant digits, then you have far too many in the first two. Since there are 3 s.f. in 5.89, you may have 3 places in the answer so it should be 0.04949 which rounds to 0.0495 moles KBr. For the second one, it is 90.687 which rounds to 91 moles to two s.f. (there are 2 s.f. in 5.3). Finally, there are 2 s.f. in 7.9 mg and the answer is 0.0000781 which rounds to 0.000078 moles KNO3.
To determine the number of moles in a substance, you need to know the molar mass of that substance. The molar mass is the mass of one mole of a substance and is expressed in grams/mol.
a. To find the number of moles in 5.89 g of KBr:
1. Find the molar mass of KBr.
- The atomic mass of K (potassium) is 39.10 g/mol, and the atomic mass of Br (bromine) is 79.90 g/mol.
- The molar mass of KBr is the sum of the atomic masses of potassium and bromine: 39.10 g/mol + 79.90 g/mol = 119.00 g/mol.
2. Use the formula:
- Number of moles = Mass / Molar mass
- Number of moles = 5.89 g / 119.00 g/mol = 0.0494 mol (rounded to four decimal places).
b. To find the number of moles in 5.3 kg of NaCl:
1. Convert the mass from kilograms to grams:
- 5.3 kg = 5300 g.
2. Find the molar mass of NaCl.
- The atomic mass of Na (sodium) is 22.99 g/mol, and the atomic mass of Cl (chlorine) is 35.45 g/mol.
- The molar mass of NaCl is the sum of the atomic masses of sodium and chlorine: 22.99 g/mol + 35.45 g/mol = 58.44 g/mol.
3. Use the formula:
- Number of moles = Mass / Molar mass
- Number of moles = 5300 g / 58.44 g/mol ≈ 90.74 mol (rounded to two decimal places).
c. To find the number of moles in 7.9 mg of KNO3:
1. Convert the mass from milligrams to grams:
- 7.9 mg = 0.0079 g.
2. Find the molar mass of KNO3.
- The atomic mass of K (potassium) is 39.10 g/mol, the atomic mass of N (nitrogen) is 14.01 g/mol, and the atomic mass of O (oxygen) is 16.00 g/mol.
- The molar mass of KNO3 is the sum of the atomic masses of potassium, nitrogen, and three oxygen atoms: 39.10 g/mol + 14.01 g/mol + (16.00 g/mol × 3) = 101.10 g/mol.
3. Use the formula:
- Number of moles = Mass / Molar mass
- Number of moles = 0.0079 g / 101.10 g/mol ≈ 0.000078 mol (rounded to six decimal places).