Transcript Document 7576409

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:
1 mole
1M =
1 Liter

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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
Or
Calculate 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