Forensic Chemistry-Mrs. Terry-McCants

Measuring matter deals with how you can convert a count, mass and mass of something.

Knowing how the count, mass and volume of an item relate to a common unit allows you to convert among these units.

For example: 1 dozen apples/12 apples 1 kg Apples/ 2.2 lbs. apples

A mole of a substance is 6.02 x 10^23 particles

Of a substance. A mole is the SI unit for measuring the

amount of a substance. Avogadro’s number is the number of

representative particles in a mole:6.02 x 10^23 The mole allows chemists to count the

number of representative particles in a substance.

Magnesium is a light metal used in the manufacture of aircraft, automobile wheels, and tools. How many moles of magnesium is 1.25 x 10^23 atoms of Mg?Steps:1.Analyze: List the knowns and the unknowns2.Calculate: Solve for the unknowns.3.Evaluate: Does this make sense?4.Refer to pg. 309 in your textbk.

Analysis: Known= 1.25 x 10^23 atoms of Mg

Unknown= moles= ? Mol Mg Calculate: Solve for the unknown and set up

as a unit conversion. Equivalent:1 mol Mg = 6.02 x 10^23 atoms

Mg 1.25 x 10^23 atoms Mg x 1 mole Mg

6.02 x 10^23 atoms of MgAns: 0.208 mol Mg

Problem 1How many moles is 2.80 x 10^24 atoms of silicon?

Problem #2How many moles is 2.17 x 10^23

representative particles of bromine?

How do you convert the mass of a substance to the number of moles of the substance?

Ans: Use the molar mass of an element or compound to convert between the mass of a substance and the moles of the substance.

Example: Molar mass and 1 mole1 Mole Molar mass

Known: # moles= 9.45 mol Al2O3

Unknown: mass= ? g Al2O3

Steps: 1. Determine the mass of 1 mole of Al2O3

2. Identify the conversion factor relating moles of Al2O3 to grams of Al2O3.

3. Multiply the given number of moles by the conversion factor.

Refer to p.318 in text sample problem 10.5.

Find the mass in grams of 2.50 moles of iron II hydroxide.

The empirical formula of a compound gives the lowest whole-number ratio of the atoms or moles of the elements in a compound.

The percent composition of a compound can be used to calculate the empirical formula of that compound.

Steps: 1. Analyze by listing your knowns &

unknowns. 2. Convert the % composition of each

element to the moles of each element. 3. Divide each molar quantity by the

smaller number of moles to get 1mol for the element with the smaller number of moles.

4. Multiply each part of the ratio by the smallest whole number that will convert both subscripts to whole numbers.

Calculate the empirical formula of each of the following:

A. 94.1% O, 5.9% H

B. 67.6% Hg, 10.8% S, 21.6% O

Steps: 1. Calculate the empirical formula mass. 2. Divide the molar mass by the empirical

formula mass. 3. Multiply the formula subscripts by this

value.

Calculate the molecular formula of a compound whose molar mass is 60 g/mol and empirical formula is CH4N.

Try #41 & #42 p.333 for additional practice.