# Forensic Chemistry-Mrs. Terry-McCants.  Measuring matter deals with how you can convert a count, mass and mass of something.  Knowing how the count,

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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 unknowns 2. 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 Mg Ans: 0.208 mol Mg

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

 Problem #2 How 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 mole 1 MoleMolar mass

 Known: # moles= 9.45 mol Al 2 O 3  Unknown: mass= ? g Al 2 O 3  Steps:  1. Determine the mass of 1 mole of Al 2 O 3  2. Identify the conversion factor relating moles of Al 2 O 3 to grams of Al 2 O 3.  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.

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