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Concentration part 1: Prelab

Objectives:

• Explore concentration and saturation as related to solvents and solutes.

• Calculate molar mass and mass of solute needed for a specific concentration.

Web link:

• PhET Concentration https://phet.colorado.edu/en/simulations/concentration

Introduction:

A mixture is a type of matter composed of varying proportions of two or more substances that are only

physically mixed and not chemically combined. Mixtures can be divided into two categories –

heterogeneous and homogeneous. A heterogeneous mixture is non-uniform so that at least two

components can be observed examples include pizza and oil/water. A homogeneous mixture is uniform

throughout like coffee or an alloy. Another name for a liquid, homogeneous mixture is a solution. A

solution is composed of two parts 1) a solvent which is the liquid or the substance in the larger quantity,

and 2) a solute which is the substance dissolved in the solvent. Figure 1 below shows two examples of

salt-water solutions. In a salt-water solution, water is in the larger amount and is the solvent, while the

salt is in the smaller amount and is the solute. At a given temperature only a certain amount of solute can

be dissolved into the solvent. When the maximum amount of solute is present the solution is said to be

saturated. Any extra solute added to a saturated solution would remain undissolved and usually settle at

the bottom of the container. When less than the maximum amount of solute is dissolved into the solvent,

the solution is called unsaturated.

Figure 1: Two Examples of Salt-Water Solutions.

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Examples:

1. In a solution which substance is present in the larger amount, the solvent or the solute?

Solvent – The solvent is the liquid or the substance in the larger quantity

2. Take a cup of water, add sugar, and stir. If the resulting solution contains sugar crystals that do not

dissolve, is the solution saturated or unsaturated?

Unsaturated - If there is no precipitate at the bottom then it is likely less than the maximum amount of

the solute is present.

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Concentration

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Moles and Molarity

In addition to the broad categories of saturated and unsaturated, solutions can be classified based on their

concentration or molarity. Molarity is the concentration of a solution expressed in the number of moles

of solute per liter of solution. In order to calculate the molarity of a solution, you need to know the

number of moles of solute and the total volume of the solution. Moles are used to measure the amount of

a substance. A mole is a number of somethings. The same way a dozen eggs means twelve eggs. A mole

of atoms is 6.02x1023 atoms. For this lab we will focus on a property of moles concerning mass. To

understand this property of moles, first we will talk about atomic mass and formula mass.

Chemical Formula and Number of Atoms

Compounds such as sodium chloride and water can be written out as words or a shorthand form of the

name can be written. This shorthand form is called the chemical formula. Look back at figure 1 and notice

that H2O is used instead of water. In the same figure NaCl is used instead of sodium chloride. The

chemical formula for a substance contains the chemical symbol for each element present. If there is more

than one atom of that element then a subscript will also be used in the chemical formula. The subscripts

show the number of atoms of that element. If there is a subscript 2, that means the compound contains two

atoms of that element. A subscript of 3 would mean the compound contains three atoms of that element.

H2O has two hydrogen atoms and one oxygen atom. NaCl has one sodium atom and one chlorine atom.

Copper(II) sulfate, CuSO4, has one copper atom, one sulfur atom, and four oxygen atoms.

The purpose of the chemical formula is to have a shorthand way of describing the compounds. Cobalt (II)

nitrate has the chemical formula Co(NO3)2. This signifies that one cobalt atom is bonded to two nitrate

ions. If we are counting atoms and parentheses are present, then we multiply the subscript times

everything in the parentheses. Therefore cobalt(II) nitrate has one cobalt atom, 2x1=2 nitrogen atoms, and

2x3=6 oxygen atoms.

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Examples:

3. The formula for potassium dichromate is K2Cr2O7. Which elements are present, and how many atoms

of each element are present in this compound?

2 K, 2 Cr, 7 O

Two atoms of potassium, two atoms of chromium, and seven atoms of oxygen

4. The formula for lead chromate is PbCrO4. Which elements are present, and how many atoms of each

element are present in this compound?

1 Pb, 1 Cr, 4 O

One atom of lead, one atom of chromium, and four atoms of oxygen

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Atomic Mass and Formula Mass

The atomic mass (AM) of an element is the average mass of all its naturally occurring isotopes, and we

can find this number on the periodic table, usually below the element’s symbol. The mass is given in the

units ‘u’. For our examples, we will round to the tenths place. We can also determine the mass of a

molecule by adding the atomic masses of its components. For example, the formula mass of sodium

chloride is the mass of one atom of sodium (23.0 u) plus the mass of one atom of chlorine (35.5 u) so the

formula mass of sodium chloride is 58.5 u.

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Example:

5. Find the formula mass (FM) of lead chromate, PbCrO4. Use the periodic table in Appendix A of this lab

Concentration

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to look up the atomic masses of the elements.

323.2 u - Lead chromate is made up of 1 atom of Pb, 1 atom of Cr, and 4 atoms of O. The atomic

masses of those elements are: Pb (207.2 u), Cr (52.0 u), O (16.0 u)

Therefore, the formula mass of PbCrO4 is 207.2 u + 52.0 u + (4 × 16.0 u) = 323.2 u

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Concentration and Moles

Now that we know what a formula mass is, we can talk about the mole again. A mole of any substance

has a mass equal to the same number of grams as its formula mass (FM). For example, copper has a FM

of 63.5u. Therefore, a mole of Cu has a mass of 63.5 grams, and copper has a molar mass of 63.5 g/mol.

Water (H2O) has a FM of 18.0u, and a mole of H2O has a mass of 18 grams. Therefore, water has a molar

mass of 18 g/mol.

If the concentration of a solution is 1 mol/L, then there is 1 mole of the solute for every liter of solution.

Sometimes chemists need to make solutions with specific concentrations such as 2.4 mol/L or 7.6 mol/L.

To make the solution, first they find the mass of the solute needed, using the relationship between mass

and moles.

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Example:

6. What mass of PbCrO4 should be added to 1 L of water to create a solution with a concentration of 3.2

mol/L?

1034.2 grams of PbCrO4

First, find the formula mass (FM) of lead chromate, PbCrO4. It is 323.2 u (see Example 5). Remember

that the formula mass in units u is equal to the mass of 1 mole of the substance in grams. Therefore 1

mole of PbCrO4 has a mass of 323.2 grams.

Next recognize that a solution with concentration 3.2 mol/L would require 3.2 moles of the solute

added to 1 L of the water.

Then find the mass for 3.2 moles of PbCrO4. Multiply the number of moles by the molar mass of the

solute.

3.2 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑃𝑏𝐶𝑟𝑂4 ( 323.2 𝑔 𝑃𝑏𝐶𝑟𝑂4

1 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑃𝑏𝐶𝑟𝑂4 ) = 1034.2 𝑔 𝑃𝑏𝐶𝑟𝑂4

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Concentration

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Appendix A