Homework

1
Homework

• Chapter 0 - 0, 1, 2, 4
• Chapter 1 15, 16, 19, 20, 29, 31, 34

2
Question

• What is the molarity of a 10 (w/v) solution of
  glucose?

3
Parts per million (PPM)
4
PPM

• Parts per million is a convenient way to express
  dilute concentrations. Historically, 1 mg per
  liter or per 1000 ml is referred to as 1 ppm.
  However, this is not really the case, as parts
  per million should be expressed as

Show that the above equation is equivalent to mg
per liter.
5
PPM
For dilute solutions, the density of the solution
will be the same as water.
Density of solution Density of water
1.0 g/ml
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Question Converting PPM to Molarity

• The town of Canton prohibits the dumping of
  copper solutions that have concentrations greater
  than 0.3969 ppm. When cleaning the quant lab,
  Dr. Skeels found a bottle labeled copper
  standard - 7 mM, is it permissible to dump this
  solution down the drain?

Volunteers??
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Preparation of Stock Solutions

• Solids
• Liquids

8
Solution preparation contd

• Describe the Preparation of a 500.0 mL of a
  solution that contains 8.00 mM Cu2 using
  CuSO4.5H2O (MW 149.69).

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Solution preparation contd

• Describe the Preparation of a 500.0 mL of a
  solution that contains 8.00 mM Cu2 using
  CuSO4.5H2O (MW 149.69).

Thus
10

• Add ______g CuSO4.5H2O
• Into a volumetric flask
• Add about _____ ml of water
• Swirl to dissolve
• And fill to the _____ ml mark

11
Question

• Using the 8 mM Cu2 solution, prepare 20 mL of a
  0.25 mM Cu2 solution.

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Dilutions

• To make dilutions of a solution, the following
  equation should be employed

13
Question

• Using the 8 mM Cu2 solution, prepare 20 mL of a
  0.25 mM solution.

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From a liquid consider concentrated HCl
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A more difficult example

• Prepare a 500.0 mL of 1 M HCl.

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MW
Wt
Density
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Try it out

• Consider it in two steps
• (1) Determine concentration of Stock
• (2) Make dilution

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(1) Concentration of Stock

• Must find grams of HCl per liter of solution

dHCl1.19 g/ml
HCl (w/w)37
MW36.46 g/mol
Mass HCl per Liter
Molarity
19
Dilution

• Determined concentration of stock is ______ M
  HCl. We want a 500.0 mL solution that is 1M.

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NOTE

• Care must be exercised
• when handling strong acids!!
• (Always, Always add acid to water)
• Add about 300 ml of water first
• Then add acid
• Dilute to mark

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Homework

• Chapter 0 - 0, 1, 2, 4
• Chapter 1 15, 16, 19, 20, 29, 31, 34

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Chapter 3

• Experimental Error
• And propagation of uncertainty

23
Suppose

• You determine the density of some mineral by
  measuring its mass
• 4.635 0.002 g
• And then measured its volume
• 1.13 0.05 ml

What is its uncertainty?
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Significant Figures (contd)

• The last measured digit always has some
  uncertainty.

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3-1 Significant Figures

• What is meant by significant figures?
• Significant figures minimum number of digits
  required to express a value in scientific
  notation without loss of accuracy.

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Examples

• How many sig. figs in
• 3.0130 meters
• 6.8 days
• 0.00104 pounds
• 350 miles
• 9 students

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Rules

• All non-zero digits are significant
• Zeros
• Leading Zeros are not significant
• Captive Zeros are significant
• Trailing Zeros are significant
• Exact numbers have no uncertainty
• (e.g. counting numbers)

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Reading a scale
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What is the value?
When reading the scale of any apparatus, try to
estimate to the nearest tenth of a division.
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3-2Significant Figures in Arithmetic

• We often need to estimate the uncertainty of a
  result that has been computed from two or more
  experimental data, each of which has a known
  sample uncertainty.
• Significant figures can provide a marginally good
  way to express uncertainty!

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3-2Significant Figures in Arithmetic

• Summations
• When performing addition and subtraction report
  the answer to the same number of decimal places
  as the term with the fewest decimal places
• 10.001
• 5.32
• 6.130

21.451
21.451
___ decimal places
?
32
Try this one

• 1.632 x 105
• 4.107 x 103
• 0.984 x 106

0.1632 x 106 0.004107 x 106 0.984 x
106


1.151307 x 106
1.151307 x 106
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3-2Significant Figures in Arithmetic

• Multiplication/Division
• When performing multiplication or division report
  the answer to the same number of sig figs as the
  least precise term in the operation
• 16.315 x 0.031

?
0.505765
___ sig figs
___ sig figs
____ sig figs
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3-2Logarithms and Antilogarithms

• From math class

log(100) 2 Or log(102) 2 But what about
significant figures?
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3-2Logarithms and Antilogarithms
Lets consider the following An operation
requires that you take the log of 0.0000339.
What is the log of this number?

• -4.469800302

• log (3.39 x 10-5)

• Between -5 and -4

• log (3.39 x 10-5)

• log (3.39 x 10-5)

• ____ sig figs

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3-2Logarithms and Antilogarithms

• Try the following
• Antilog 4.37

• 23442

• 2.3442 x 104

___ sigs
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Rules

• Logarithms and antilogs
• 1. In a logarithm, keep as many digits to the
  right of the decimal point as there are sig figs
  in the original number.
• 2. In an anti-log, keep as many digits are there
  are digits to the right of the decimal point in
  the original number.

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3-4. Types of error

• Error difference between your answer and the
  true one. Generally, all errors are of one of
  three types.
• Systematic (aka determinate) problem with the
  method, all errors are of the same magnitude and
  direction (affect accuracy)
• Random (aka indeterminate) causes data to be
  scattered more or less symmetrically around a
  mean value. (affect precision)
• Gross. occur only occasionally, and are often
  large.

Can be detected and eliminated or lessened
Estimated
Treated statistically
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Absolute and Relative Uncertainty

• Absolute uncertainty expresses the margin of
  uncertainty associated with a measurement.
• Consider a calibrated buret which has an
  uncertainty 0.02 ml. Then, we say that the
  absolute uncertainty is 0.02 ml

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Absolute and Relative Uncertainty

• Relative uncertainty compares the size of the
  absolute uncertainty with its associated
  measurement.
• Consider a calibrated buret which has an
  uncertainty is 0.02 ml. Find the relative
  uncertainty is 12.35 0.02, we say that the
  relative uncertainty is

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3-5. Estimating Random Error (absolute
uncertainty)

• Consider the summation
• 0.50 ( 0.02)
• 4.10 ( 0.03)
• -1.97 ( 0.05)

Sy 0.06
2.63 ( ?)
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3-5. Estimating Random Error

• Consider the following operation

0.010406
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Try this one
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3-5. Estimating Random Error

• For exponents

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3-5. Estimating Random Error

• Logarithms antilogs

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Question

• Calculate the absolute standard deviation for a
  the pH of a solutions whose hydronium ion
  concentration is
• 2.00 ( 0.02) x 10-4
• pH 3.6990 ?

47
Question

• Calculate the absolute value for the hydronium
  ion concentration for a solution that has a pH of
  7.02 ( 0.02)
• H 0.954992 ( ?) x 10-7

48
Suppose

• You determine the density of some mineral by
  measuring its mass
• 4.635 0.002 g
• And then measured its volume
• 1.13 0.05 ml

What is its uncertainty?
49
The minute paper

• Please answer each question in 1 or 2 sentences
• What was the most useful or meaningful thing you
  learned during this session?
• What question(s) remain uppermost in your mind as
  we end this session?