The mole is the standard unit of measurement in chemistry and this unit takes into account the different elements present in a chemical compound. Often times, the amount of compounds is stated in grams and must be converted to moles. This conversion can help give you a clearer picture of the number of molecules you're working with rather than dealing with weight, which can change between molecules. Although the conversion is simple, there are a couple of very important steps that you must follow. Using this method, you will learn how to convert from grams to moles.
Part 1 of 2: Calculate Molecular Mass
Step 1. Gather the tools you need to solve a chemistry problem
Having everything you need at your fingertips will simplify the process of solving the assigned problem. You need the following:
- A pencil and paper. Calculations are easier to solve when written. Make sure you write down all the steps to get a good grade.
- A periodic table. You need to be able to find the atomic weight of the elements using the periodic table.
- A calculator. Calculators are necessary to simplify complex number calculations.
Step 2. Identify the elements in the compound that you need to convert to moles
The first step in calculating molecular mass is to identify each element that makes up the component. It is very easy to distinguish the elements since the abbreviations contain only one or two letters.
- If a compound is abbreviated in two letters, the first will be in uppercase and the second in lowercase. For example, "Mg" is short for "magnesium."
- The compound NaHCO3 It is made up of four elements: sodium (Na), hydrogen (H), carbon (C) and oxygen (O).
Step 3. Determine the number of atoms that each element contributes to the compound
You must know how many atoms of each element are present to calculate the molecular mass. The number of atoms of each element are written as a subscript next to that element.
- For example, H2Or it has two hydrogen atoms and one oxygen atom.
- If a compound has a parenthesis followed by a subscript, each item within that parenthesis is multiplied by the number in the subscript. For example, (NH4)2S has two "N" atoms, eight "H" atoms, and one "S" atom.
Step 4. Write the atomic weight of each element
A periodic table is the easiest way to find the atomic weight of an element. Once you locate the element in the table, the atomic weight will usually be found under the symbol for that element.
- Atomic weight, mass, or an element will be given in atomic mass units (amu).
- For example, the molecular weight of oxygen is 15.99.
Step 5. Calculate the molecular mass
The molecular mass of a substance is calculated as the number of atoms of each element multiplied by the atomic weight of that element. Knowing the molecular mass is necessary to convert grams to moles.
- Multiply the number of atoms each element contributes to the compound by the atomic weight of that element.
- Add up the total weight of each item in the composite.
- For example, (NH4)2S has a molecular weight of (2 x 14.01) + (8 x 1.01) + (1 x 32.07) = 68.17 g / mol.
- Molecular mass is also known as molar mass.
Part 2 of 2: Converting from grams to moles
Step 1. Establish the conversion formula
The number of moles a compound has that can be calculated by dividing the number of grams of the compound by the molecular mass of the compound.
The formula looks like this: moles = grams of the compound / molecular mass of the compound
Step 2. Enter the numbers in the formula
Once you have the formula, the next step is to write your calculations in the correct part of the formula. An easy way to check that everything is in order is by units. Canceling the units should leave you with only moles.
Step 3. Solve the equation
Using a calculator, divide the number of grams by the molecular mass. The result is the number of moles in the element or compound.
- For example, suppose you have 2 grams of (NH4)2Yes and you want to convert them to moles. The molecular mass of (NH4)2S is 68.17g / mol. Divide 2 by 68.17 and you get 0.0293 moles of (NH4)2S.
- Always include the name of the element or compound in your answer.
- If you are asked to show your work on a chemistry assignment or test, be sure to identify your answer clearly by circling or square.