READ: Molar Mass

When creating a solution, how do I know how much of each substance to put in?

I want to make a solution that contains 1.8 moles of potassium dichromate.  I don’t have a balance calibrated in molecules, but I do have one calibrated in grams.  If I know the relationship between moles and the number of grams in a mole, I can use my balance to measure out the needed amount of material.

Molar Mass

Molar mass is defined as the mass of one mole of representative particles of a substance. By looking at a periodic table, we can conclude that the molar mass of lithium is 6.94 g, the molar mass of zinc is 65.38 g, and the molar mass of gold is 196.97 g.  Each of these quantities contains 6.02 × 10²³ atoms of that particular element.  The units for molar mass are grams per mole or g/mol.

Molar Masses of Compounds

A molecular formula of the compound carbon dioxide is CO₂.  One molecule of carbon dioxide consists of 1 atom of carbon and 2 atoms of oxygen.  We can calculate the mass of one molecule of carbon dioxide by adding together the masses of 1 atom of carbon and 2 atoms of oxygen.

12.01 amu+2(16.00 amu)=44.01 amu

The molecular mass of a compound is the mass of one molecule of that compound.  The molecular mass of carbon dioxide is 44.01 amu.

The molar mass of any compound is the mass in grams of one mole of that compound.  One mole of carbon dioxide molecules has a mass of 44.01 g, while one mole of sodium sulfide formula units has a mass of 78.04 g.  The molar masses are 44.01 g/mol and 78.04 g/mol respectively.  In both cases, that is the mass of 6.02 × 10²³ representative particles.  The representative particle of CO₂ is the molecule, while for Na₂S, it is the formula unit.

Sample Problem:  Molar Mass of  a Compound

Calcium nitrate, Ca(NO₃)₂, is used as a component in fertilizer Determine the molar mass of calcium nitrate.

Step 1:  List the known and unknown quantities and plan the problem.

Known

• formula = Ca(NO₃)₂
• molar mass Ca = 40.08 g/mol
• molar mass N = 14.01 g/mol
• molar mass O = 16.00 g/mol

Unknown

• molar mass Ca(NO₃)₂

First we need to analyze the formula.  Since the Ca lacks a subscript, there is one Ca atom per formula unit.  The 2 outside the parentheses means that there are two nitrate ions per formula unit and each nitrate ion consists of one nitrogen atom and three oxygen atoms.  Therefore, there are a total of 1 × 2 = 2 nitrogen atoms and 3 × 2 = 6 oxygen atoms per formula unit.  Thus, 1 mol of calcium nitrate contains 1 mol of Ca atoms, 2 mol of N atoms, and 6 mol of O atoms.

Step 2:  Calculate.

Use the molar masses of each atom together with the number of atoms in the formula and add together.

1 mol Ca×40.08 g Ca1 mol Ca=40.08 g Ca2 mol N×14.01 g N1 mol N=28.02 g N6 mol O×16.00 g O1 mol O=96.00 g Omolar mass of Ca(NO3)2=40.08 g+28.02 g+96.00 g=164.10 g/mol

Summary

• Calculations are described for the determination of molar mass of an atom or a compound.

CC-BY-NC http://www.ck12.org/chemistry/Molar-Mass/lesson/Molar-Mass-CHEM/?referrer=featured_content