Calculating mass in chemistry often involves determining the mass of individual atoms, molecules, or compounds. There are several ways to do this, depending on what you are given and what you need to find. This response focuses on calculating the molar mass (also called molecular weight or formula weight), which is the mass of one mole of a substance, and using molar mass to convert between mass and moles.
1. Calculating Molar Mass
Molar mass is the mass of one mole of a substance (element, molecule, or compound) and is expressed in grams per mole (g/mol). Here's how to calculate it:
Step 1: Identify the Chemical Formula
Determine the chemical formula of the substance. For example:
- Water: H2O
- Sodium Chloride: NaCl
- Glucose: C6H12O6
Step 2: Find the Atomic Masses
Locate the atomic mass of each element in the chemical formula on the periodic table. Atomic masses are usually found below the element symbol. Round these masses to a reasonable number of decimal places (usually two is sufficient). For example:
- Hydrogen (H): 1.01 g/mol
- Oxygen (O): 16.00 g/mol
- Sodium (Na): 22.99 g/mol
- Chlorine (Cl): 35.45 g/mol
- Carbon (C): 12.01 g/mol
Step 3: Multiply and Add
Multiply the atomic mass of each element by the number of atoms of that element present in the chemical formula, and then add up the results.
Example 1: Water (H2O)
- (2 atoms of H) * (1.01 g/mol) = 2.02 g/mol
- (1 atom of O) * (16.00 g/mol) = 16.00 g/mol
- Molar mass of H2O = 2.02 g/mol + 16.00 g/mol = 18.02 g/mol
Example 2: Sodium Chloride (NaCl)
- (1 atom of Na) * (22.99 g/mol) = 22.99 g/mol
- (1 atom of Cl) * (35.45 g/mol) = 35.45 g/mol
- Molar mass of NaCl = 22.99 g/mol + 35.45 g/mol = 58.44 g/mol
Example 3: Glucose (C6H12O6)
- (6 atoms of C) * (12.01 g/mol) = 72.06 g/mol
- (12 atoms of H) * (1.01 g/mol) = 12.12 g/mol
- (6 atoms of O) * (16.00 g/mol) = 96.00 g/mol
- Molar mass of C6H12O6 = 72.06 g/mol + 12.12 g/mol + 96.00 g/mol = 180.18 g/mol
2. Using Molar Mass to Convert Between Mass and Moles
The molar mass acts as a conversion factor between mass (in grams) and amount (in moles).
Converting Moles to Mass
To convert moles to mass, multiply the number of moles by the molar mass:
Mass (grams) = Moles * Molar Mass
Example: How many grams are in 0.5 moles of NaCl?
- Moles of NaCl = 0.5 mol
- Molar Mass of NaCl = 58.44 g/mol
- Mass of NaCl = 0.5 mol * 58.44 g/mol = 29.22 g
Converting Mass to Moles
To convert mass to moles, divide the mass by the molar mass:
Moles = Mass (grams) / Molar Mass
Example: How many moles are in 100 grams of H2O?
- Mass of H2O = 100 g
- Molar Mass of H2O = 18.02 g/mol
- Moles of H2O = 100 g / 18.02 g/mol = 5.55 mol
These calculations are fundamental in stoichiometry and many other areas of chemistry. Understanding how to calculate molar mass and use it for conversions is crucial for solving a wide variety of problems.
