units for quantities and concentration

Topic 1

Units for Quantities and

Concentration

12 March 2012 DrSabiha/CHM421/March-July2012 1

Analytical chemistry is a measurement

science consisting of a set of powerful

ideas and methods that are useful in all

fields of science and medicine.

INTRODUCTION

What is analytical chemistry?


©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

How is it important and relevant to

you?

• The techniques and tools of analytical

chemistry play a vital role in many areas of

science and “modern” life including

biology, medicine, environmental science,

forensics, toxicology, pharmacy etc.



4th January 2011 DrSabiha/CHM421/Dec 10-Apr11 4


QUALITATIVE

Analysis conducted to identify what are the constituents present in the sample (identification of the sample component)

QUANTITATIVE

Analysis to determine how much of each constituent present in the sample.

QUALITATIVE or QUANTITATIVE analysis


Qualitative analysis is what.

Quantitative analysis is how much.


4th January 2011 DrSabiha/CHM421/Dec 10-Apr11 7

1. Understanding and Defining

the Problem

• What accuracy is required?

• Is there a time (or money) limit?

• How much sample is available?

• How many samples are to be analyzed?

• What is the concentration range of the analyte?

• What components of the system will cause an

interference?

• What are the physical and chemical properties of the

sample matrix? (complexity)

2. History of sample and background

of the problem

Background info can originate from many sources:

• The client, competitor’s products

• Literature searches on related systems

• Sample histories:

• synthetic route

• how sample was collected, transported, stored

• the sampling process

Performance Characteristics: Figures of Merit

Which analytical method should I choose? How good

is the measurement, information content

How reproducible is it? Precision

How close to the true value is it? Accuracy/Bias

How small of a difference can be measured?

Sensitivity

What concentration/mass/amount/range? Dynamic

Range

How much interference? Selectivity (univariate vs.

multivariate)

3. Plan of Action

4. Analyzing and Reporting Results

• Analytical data analysis takes many forms: statistics,

chemometrics, simulations, etc…

• Analytical work can result in:

• peer-reviewed papers, etc…

• how sample was collected, transported, stored

• technical reports, lab notebook records, etc...

Components of an Analytical Method

Perform measurement

(instrumentation)

Handbook, Settle

Compare results

with standards

Pretreat and prepare sample

Obtain and store sample

Apply required

statistical techniques

Verify results

Present information

Extract data

from sample

Covert data

into information

Transform

information into

knowledge

After reviewing results

might be necessary

to modify and repeat

procedure

6.023x1023 is known as Avogadro’s number, N

Mole

Mole can be defined as the number of elementary entities

of a system (e.g atoms, molecules, ion, etc) that contain the

same number of atom exist in exactly 12.0 gram Carbon-

12


Atomic Weight

The average mass of atoms of an element , calculated using

the relative abundance of isotopes in a naturally-occurring

element.

It is the weighted average of the masses of naturally-

occurring isotopes.

Also Known As: atomic Mass , Examples: The atomic mass

of carbon is 12.011; the atomic mass of hydrogen is 1.0079


Gram atomic weight

The mass in gram of any element which is numerically

equal to its atomic weight is called gram atomic weight

GAW, e.g. GAW of 12C is equal to 12.0g.


Molecular weight (MW)

Molecular weight is the sum of the average atomic weight of all

the atoms in a molecule. Also same with the molar mass

(MM) or Molecular mass (MM)

Formula weight

In general for any compound, the term use is interchangeably

with formula weight when reference is made to molecular

(non ionic) substance that exist as discrete molecules.


SI units known as Internal System of Unit

Physical Quantity Name of unit Abbreviation

Mass kilogram kg

Length meter m

Time second s

Temperature kelvin K

Amount of substance mole mol

Electric current ampere A

Luminous intensity candela cd


Number of moles A = mass of A (g) molar mass (g/mol)

Number of mmoles A = mass of A (mg) molar mass (mg/mmol)

Mole

Milimole

103 milimole is equal to 1.0 mole

or

1 milimole is 1/1000 of a mole.


Example 1: How many moles and milimoles of

benzoic acid (M=122.1g/mol) are contained in 2g

of pure acid.

Example 2: Calculate the number of moles and

milimoles in the following chemical compounds, 218

g of baking soda, NaHCO3.


Example 3: Determine the mass in gram of Na+

(23 g/mol) in 25 g of Na2SO4 (142g/mol).

( Ans: 8.098 g )


Solution concentration (Molarity , Normality)

Concentration?

Concentration is the amount of solute in a given volume of

solution.

Solvent : A substance that generally a liquid present in the

larger proportion of the solution. Water is considered a

universal solvent for most solution.

Solute : the substance present in a smaller proportion of a

solution. It can be solid, liquid or gas.

Concentration (C ) = mass (g)

volume (L)


Example 1:

Calculate the concentration of H2SO4 solution when

98g of H2SO4 is dissolved in 1.0 liter of water

Example 2 :

The concentration of solution B is 12.1 g/L.

Calculate the weight of solute present in 2.23 L of

its solution.


The number of moles of solute per liter of

solution, given the symbol , M.

M = no of moles of solute

volume of solution (L)

or

= no of milimoles of solute

volume of solution (mL)

Molarity


Example 3: What is the molarity of a solution made by

dissolving 2.5 g of NaCl in enough water to make 125

ml of solution?


Example 4: How many mole of solute present in the

following solutions:

i) 16.3 liter solution 0.113 M.

(Ans: 1.8419 mole)

ii) 15.6 ml solution 0.025 M.

(Ans: 0.00039 mole)


1. A chemist dissolves 98.4 g of FeSO4 in enough

water to make 2.000 L of solution. What is the

molarity of the solution? (Ans 0.324M)

2. How would you prepare 100. mL of 0.25 M KNO3

solution? (Ans 2.53 g KNO3)

Exercises


Number of moles of solute that dissolved in 1000

g of solvent , m .

Preferred expression of concentration involving colligative properties

(b.p elevation, freezing point depression, osmotic pressure).

Molality

m = no of mol of solute

1 kg of solvent


Number of gram equivalents of reacting unit

per liter of solution, given the symbol , N

N = no of equivalents

volume (L)

Normality

No. of reacting unit = No. of equivalent


For 1 mol of sulfuric acid, H2SO4 has 2 reacting units

of protons

Equivalent weight = FW no. reacting units

= (98.08 g/mol) (2 eq/mol)

= 49.04 g/eq

For 1 mol, 1L of H2SO4, No. equivalent = 98.08 49.04

= 2

Normality = 2 1L, Molarity = mol vol(L)

= 2 N = 11

= 1 M


Stoichiometry

Stoichiometry is defined as the mass relationships

among reacting chemical species.

For the reaction

2N2H4 + N2O4 3N2 + 4H2O, determine

1. the number of moles of N2O4 required to react with 2.72 moles of N2H4.

2. the number of grams of N2 produced when 2.72 moles of N2H4 are consumed.

3. the mass in grams of H2O formed when 1.00 g of N2O4 reacts.


Chemical Stoichiometry

Stoichiometry: The mass relationships among reacting

chemical species. The stoichiometry of a reaction is the

relationship among the number of moles of reactants

and products as shown by a balanced equation.

Mass Moles Moles Mass

Divide by molar mass

Multiply by stoichiometric

ratio Multiply by molar mass

units for quantities and concentration

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