Volumetric Analysis

• Frequently, we will react 2 solutions with each other• These reactions are called titrations:

– Water analysis– Environmental Sciences– Food industry– Medical laboratories

• Typical titrations involve reacting acids and bases– We add an indicator (ie: phenolphthalein) that

changes color when the titration is complete• The concentration of the analyte is determined by

measuring how much of the standard material of known concentration was used

Approach is the same, but instead of working with solids, we have solutions

Remember the definition of Molarity:

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1M = 1 mole

1 Liter

In a titration, 3.25g of acid (HX) requires 68.8 mL of 0.750M NaOH for complete reaction. What is the molar mass of the acid?

Step 1: Balanced Chemical reaction

Step 2: Calculate the # of moles of NaOH

Step 3: Determine the # of moles of HX

Step 4: Calculate the molar mass of HX

Suppose that 25.0mL of 0.5M K2CrO4(aq) reacts with 15.0 mL of AgNO3(aq) completely. What mass of NaCl is needed to react completely with 35.0mL of the same AgNO3 solution?

Step 1: Balanced chemical equation

Step 2: Determine the # of moles of K2CrO4

Step 3: Determine the # of moles of AgNO3

Step 4: Determine the Molarity of the AgNO3 solution

Step 5: Reaction between AgNO3 and NaCl

Step 6: Determine the # of moles of AgNO3 in 35.0 mL

Step 7: Calculate the NaCl mass in that # of moles

Fundamentals M: Limiting Reagents

• We have only considered reactions where the exact amounts of reactants were present

• In this chapter, we’re going to look at more realistic reactions and determine how to ascertain the extent of reaction based upon the amount of reactants present

• First, we’ll need to define some terms…

Reaction Yield

1. The Theoretical Yield is the maximum quantity of products that can be obtained from a given amount of reactant

2. The Percentage Yield is equal to the Actual Yield divided by the Theoretical Yield multiplied by 100.

• Many “real world” experiments are reported in terms of percent yield

In an automotive engine, 1.00L of octane (C8H18) (702g of octane) is burned but only 1.84 kg of carbon dioxide is produced. What is the percentage yield of carbon dioxide?

Step 1: Balanced chemical reaction

Step 2: Calculate the number of moles of octane

Step 3: Calculate the # of moles of CO2 that should be made (theoretical yield)

Step 4: Calculate the actual moles of CO2 made

The Limiting Reactant

• A limiting reactant is the reactant that governs the maximum yield of product obtained.

• The limiting reactant is consumed before the other reactant(s) are and makes the reaction stop.

• In order to determine the limiting reactant, we must know the balanced chemical equation

How do we determine the limiting reactant?

• Step 1: You’ll be given something (mass or # of moles) of the reactant. Convert to moles

OrCalculate the # of moles of each reactant you

actually have• Step 2: Look at the balanced chemical equation.

Find the reactant with the highest stoichiometric coefficient and divide it by the stoichiometric coefficient of the other reactant.

Now, do the same for the # of moles of reactant you calculated in Step 1.

•Step 3: Look at the values from Step 2.

Does the last calculation (of the actual # of moles) give you a higher number than the calculation with the stoichiometric coefficients? If so, the reactant with the lower stoichiometric coefficient is limiting, if not, then the reactant with the higher stoichiometric coefficient is limiting

•Step 4: The # of moles of the limiting reactant determines the maximum amount of product that can be formed.

Alright, that was really long. Let’s look at some examples for help…

If 28g of NO2 and 18g of H2O are allowed to react to form nitric acid and nitrogen monoxide, and 22g of nitric acid is produced in the reaction, what is the percentage yield?

Step 1: Determine the limiting reactant

Step 2: Calculate the theoretical # of moles of product

Step 3: Calculate the actual # of moles of product

Step 4: Calculate the % yield

In the synthesis of ammonia, what is the percentage yield of ammonia when 100 kg of hydrogen reacts with 800 kg of nitrogen to produce 400 kg of ammonia.

Step 1: Balanced chemical equation

Step 2: Calculate the # of moles of reactants

Step 3: Identify the limiting reactant

Step 4: Calculate the theoretical yield based upon the limiting reactant

Step 5: Calculate the actual # of moles of product

Step 6: Calculate the % yield

Combustion Analysis• One of the most important techniques in the analysis of an

unknown organic compound is Combustion Analysis

• In this technique, an organic sample is combusted in a furnace and the products are quantitated in a gas chromatograph

Combustion Analysis

CxHyOz + O2 --> CO2 + H2O

• All the hydrogen in water comes from the unknown sample• All the carbon in carbon dioxide comes from the unknown

sample• We will know the mass of the unknown compound and the

mass of the products– We use the Law Of Conservation of Mass and these values to

calculate the amount of oxygen (or other atom) in the unknown sample.

– We can then use other information given to calculate the molecular formula

A combustion analysis was carried out on 1.621g of a newly synthesized compound, which was known to contain only C, H and O. The masses of water and carbon dioxide produced were 1.902g and 3.095g, respectively. What is the empirical formula of the compound?

Step 1: Determine the amounts of H and C in the products

Step 2: Using the Law of Conservation of Mass, determine the amount of oxygen in the compound

Step 3: Determine the # of moles just like we did in the mass percentage problems you know and love

When 0.528g of sucrose (which consists of carbon, hydrogen and oxygen) is burned, 0.306g of H2O and 0.815g of CO2 are formed. What is the empirical formula of the compound?

Step 1: Determine the amounts of H and C in the products

Step 2: Determine the amount of oxygen in the sample

Step 3: Calculate the ratios of the moles of the atoms