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Midterm

# Chem1100a Midterm review up to chapter 2

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Western University

Chemistry

Chemistry 1027A/B

Kay Calvin

Fall

Description

Chem1100A T est
1 Review
2012/2013o you by the Science Soph Team General Testing Tips • Know definitions of what something is
• What is a node?
• What is an electron
• Think about what you are given in the question and
what you need to find
• Use units, write them out
• Make sure you finish the question, often a MC answer
will be the incomplete step
Studying Tips • Read
• Don’t look at answer
• Write out what you are given
• Write out what you need to find
• Solve
• Check the other answers and write why each is
incorrect
How to attack the
question • The 10 questions you guys got wrong last year
Practice Test Questions Question #1 Section from Text • First, we want to write down our known variables.
• Density of solution = 1.65 g/mL
• Solution = 32.7 % glucose by WEIGHT* (this is key!)
• Step 1: Since we are given the percent by weight, we should
find the mass of glucose in solution. (Whenever there is no
solution VOLUME given, work with the easiest numbers you
can. And since the final part of this question involves
molarity (mol/Liter), why not just choose 1 Liter?
• Solution density: 1.65 g / 0.001 L = x g / 1 L
… therefore, x = 1650 g/L
Our Solution • Step 2: So our 1 Liter solution weighs 1650 g. 32.7% of that, or
540 grams, is glucose. There are 540 grams of glucose per L
of solution.
• Molar mass of glucose (C H O ) is 180 g/mol.
6 12 6
• Step 3: Find out how many moles of glucose are in 1 Liter of
solution (which is essentially “the molarity of the glucose
solution”.
• 540 g / x mol = 180 g / 1 mol
… therefore x = 3 moles of glucose in 1 L
Therefore, the molarity of the solution is 3.00 mol/L (or 3 M).
Our Solution Lee’s Solution (found on
WebCT) Question #2 You have the pH
so you can use
that in an
equation to get
The question the concentration
then use that to
is asking find mols then
you how finally grams
which is what you
many are looking for.
grams of
CaCO3 are
Keep your work
needed to organized so you
raise the don’t get mixed up.
Remember that you
pH from 1.5 are looking for CaCO3
to 2.6 - it is common to
convert to grams
after you find mols of
H+ but that’s not
what is being asked!
Our Solution Lee’s Solution (found on
WebCT) Question #3 • First we need to know that a molecule with a Permanent Dipole Moment: is a molecule
differences in electronegativity , resulting in one area attracting more electrons andto
becoming negative, and the other becoming positive.
• Next, we need to understand the rules for drawing Lewis structures as well as VSEPR
theory.
Our Solution Rules for Drawing Lewis Structures (Calvin)
1. Calculate total number of valence electrons for your molecule
2. Add/subtract if your molecule is an ion (if it's a cation, you subtract electrons; if it's an
anion, you add electrons)
3. Draw the structure with just the central atom having one bond to each surrounding
molecule. Subtract 2 times the number of bonds from the total number of electrons
4. Add the remaining electrons as lone pairs around the more electronegative atom
(usually the outer atoms) and then less electronegative if you still have some left over
5. Add charges to the atoms, if necessary
6. Add double/triple bonds if necessary (usually if there is a positive charge on one atom
and a negative on an adjacent one, it means you can move one of the lone pairs on the
negative to form a double bond)**
7. You are done
*Make sure to follow the octet rule, cause otherwise it can seriously fuck you up
Our Solution • BF3
CF3-
Our Solution XeF4
Our Solution • SO3
Our Solution • IF2
Our Solution Lee’s Solution (found on
WebCT) Question #4 Our Solution Our Solution Lee’s Solution (found on
WebCT) Question #5 solution.tand what is being asked of you – to find the pH of the
2) Even if you don't know where to start, you should look forunits
that are common in the different values you are given.
To-do list:
Density Mass Quantity Concentration pH
So to begin, notice 1.84 g/mL and 10.0 mL, which both have the mL
unit. Multiplying these two will give us a number in grams:
1.84 g/mL x 10.0 mL
= 18.4 g
Our Solution • The question mentions the word mass when they say
that the concentrated solution is 95.02 H 4O by mass.
And just recently you found the mass of the solution
(18.4 g) by multiplying volume and density. So,
18.4 g x 0.95
= 17.48 g
Our Solution • Remember that we've been working to find pH this whole
time and a number expressed in grams won't do you much-
we want to get a H concentration so that we can take the
negative log of it to reach our answer in pH.
• Finding the quantity is the next step on our “to-do” list:
n = m/M
= 17.48 g / 98 g/mol
= 0.1784 mol
Our Solution • You're not quite done yet because although you found the
quantity of H 2O ,4it's the H ions that we take the negative
log of to find our pH. Simply look at what H 2O w4ll
dissociate into to find the proportion of H+ ions that will
come out of it:
• H SO --> 2 H + SO4 2-
2 4
• THIS IS WHAT MOST PROFESSORS WOULD CALL A

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