1 Molecular Mass & Mr. ShieldsRegents Chemistry U04 L01

2 Atomic Mass Units Recall all Atomic masses are based on Carbon-12 - C-12 is Defined to be equal to EXACTLY 12 amu’s - 1 amu = 1/12 the mass of carbon-12 - This defines a SCALE that is used to specify the mass of all other elements relative to C-12 Note: 1) Atomic mass and Mass # are different. 2) Atomic mass is also called atomic wt.

3 Using this scale, C-12 = exactly amu but… - The Atomic mass of Chlorine-35 = amu The Atomic mass of Chlorine-37 = amu - The Atomic mass of each element is the weighted average of all it’s isotopes - For Ex. the Atomic mass of Chlorine is and the atomic mass of Carbon is i.e the weighted avg of C-12, C-13, C14 Weighted Avg

4 Molecular Compounds Do you remember the definition Compounds? - Two or more elements joined together by chemical bonds - Compounds can exist in either of two forms: MOLECULES or IONIC COMPOUNDS -Molecules are a non-repeating single neutral group of atoms covalently bonded together (this means diatomic elements are also molecules) Ex. of molecules: N 2, C 6 H 12 O 6, C 6 H 14, C 2 H 5 OH, CO 2, or H 2 O

5 IONIC COMPOUNDS Ionic compounds are REPEATING representative units of Positive (cations) and Negative ions (anions) joined together to form a neutral compound =

6 Representative Unit  The unit in a compound depends on the structure of the substance.  For covalently bonded substances, the representative unit is the molecule (non-repeating).  For ionic bonded atoms, which make a crystal lattice, the representative unit is the repeating formula unit, ex. NaCl

7 Molecular Mass Let’s look at a molecule of water … How many H atoms are there and what is it’s total Atomic mass ? 2 Hydrogen = amu x 2 = amu How many O atoms are there and what’s it’s total Atomic mass? 1 Oxygen = amu

8 Molecular Mass Now let’s add them together: 2 Hydrogen = amu + 1 Oxygen = amu amu The sum of all the masses of the atoms in a molecule expressed in amu’s is called the MOLECULAR MASS Calculate the molecular mass of C 6 H

9 Formula Mass Remember … Molecules exist as single entities while Ionic compounds exist as Repeating units of Positive & Negative ions in a crystal lattice. (so NaCl is actually Na x Cl x where x= some large number). Molecules can thus be easily & accurately represented by The molecular mass ( ex. Sugar - C 6 H 12 O 6 = amu) Whereas IONIC (Crystalline) compounds can not. Why?

10 Formula Mass Problems What is the FORMULA MASS for CaF 2 ? (Ca) (2F) = amu Instead of Molecular mass we need to use something called FORMULA MASS. Formula mass is the mass of one single repeating unit.

11 AMU to Mass Up to now we’ve discussed Atomic mass in terms of AMU’s. How can we relate AMU’s to something we can weigh in the Laboratory, like grams? Let’s consider the ATOMS 1 H and 14 N a.1 atom of 1 H = 1 amu (rounded) b.1 atom of 14 N = 14 amu (rounded) c.What is the ratio of AMU’s? 1 : 14

12 AMU to Mass What happens to the ratio if we increase the number of 1 H and 14 N atoms equally? Nothing. Let’s take a look … No. of Atoms Mass of H (amu’s) Mass of N (amu’s) Ratio of amu’s 11141: : : :14

13 AMU to Mass When comparing elements, as long as the ratio of amu is held constant then the number of atoms present For each element must be equal. THE NEXT QUESTION YOU MIGHT ASK IS… How many atoms would be necessary to equal in grams an atoms atomic mass in amu’s?

14 AMU to Mass For example the Atomic mass of Sn is amu’s How many atoms are needed so that all the atoms collectively weigh grams? From this the concept of the MOLE was eventually developed.

15 AMU to Mass Scientists have experimentally determined that for C-12 the number of atoms needed to equate to 12 amu to 12 grams is: 6.02 X (1 mole) or 6 02,000,000,000,000,000,000,000 Atoms A mole of any element is defined as the number of atoms in exactly 12.0 grams of Carbon-12.

16 AMU to MASS Recall that for any two elements there would always be an equal number of atoms of each as long as the ratio of atomic masses remained constant # of atomsMass of C-12Mass of N-14Ratio of C to N masses 112 amu14 amu1 : x (1 mole) 12 g14 g 1 : This means there are 6.02 x atoms in EVERY ATOMIC MASS (amu) value when expressed in grams !

17 If there are 6.02 x atoms in 12 grams of C-12 how Many atoms are there in 6 grams of C-12? 3.01 x atoms Problem: How many grams are there in 5.03 x atoms of Zn? (Remember: 6.02 x = # atoms needed to express amu’s as grams) (5.03 x /6.02 x ) x Atomic Mass = moles x amu = 54.63g

18 Avogadro's number = × atoms/mole 6.02 x10 23 /1x10 7 = 6.02x10 16 sec or about 2 billion years For example: If a computer could count 10 million atoms per second how long would it take to count 1 mole of atoms? This is usually designated as N A Amadeus Avogadro Today we know what this number of atoms is to a high degree of Accuracy and it has a name This is a really BIG number!

19 Remember … Molecular Mass is used for molecules And is expressed in AMU’s Formula Mass is used for Crystalline (ionic) compounds and is also expressed in AMU’s And If we express an element or compounds mass in grams It is called … The Gram Molecular Mass or the Gram Formula Mass