Activity 7

Activity Tasks:

1. Review the Content Slides Acids and Bases on the D2L site.

2. Complete the Teaching Idea “Concept Questions for Chemistry using PhET”  posted by Trish Loeblein on the pH Scale simulation at PHET (http://phet.colorado.edu/en/simulation/ph-scale). On your blog post the answers with your scientific explanations from the “Clicker Questions pH Scale” posted by Trish.
1. B. False, pH determines acidity, not color.
2. D. Because both B and C are above 7 so they are both basic.
3. C. Because there is a higher number of Hydrogen atoms.
4. B. Because there is a lower number of Hydrogen atoms.
5. D. Because both A and B have high levels of Hydrogen.
6. A. Because adding water will lessen the acidity, so the pH level goes up. 
7. B. Because adding water will lessen the basicity, so the pH level goes down, but not by 2. 
8. A. Because the numbers gradually get higher.
9. C.
10. A. Something was added to the spit to make its equilibrium shift to the left.
3. Complete the Teaching Idea “Intro to Strong and Weak Acids and Bases” posted by Chris Bires on the Acid-Base Solutions simulation (http://phet.colorado.edu/en/simulation/acid-base-solutions) and post on your blog your data and answers to the questions posed.

Name:____________________

Introduction to Strong and Weak Acids and Bases PhET Lab (rvsd 5/2011)

        How does the strength of an acid or base affect conductivity?pH?

Introduction:

When you test your pool’s pH, what are you those little vials or paper strips telling you?  When you hear an acid called “strong” or “weak”, what do those terms refer to?  In aqueous solutions, compounds can exist as molecules (undissociated) or ions (dissociated).  When an acid or a base exists in solution nearly completely as dissociated ions, we refer to that acid or base as strong.  A weak acid or base will donate ions to the solution, but will remain primarily as undissociated molecules.

 

Notation:

Acids are abbreviated HA, with the H representing the proton (H+) the acid donates to the solution.  The A is referred to as the acidic anion (A-) that is left in solution as the proton is donated.

Strong Bases are abbreviated MOH, with the OH representing the hydroxide ion (OH-) the base donates to the solution.  The M is cation (M+) that is left in solution as the hydroxide is donated..

 

Autoionization:

Even without any acid or base added a very small number of water molecules will form protons (H+) and hydroxide ions (OH-).  The protons will then form hydronium ions, the acid ion.

 

Procedure: PhET Simulations à Play With Sims à Chemistryà Acid-Base Solutions à  

The concentration of the acids and bases used in the  at 0.010 (10^-2) Molar.

 

·         Begin with a strong acid and lower the pH probe into the beaker.  What is the pH of this solution? 

·         Test this strong acid with both pH paper and the conductivity probe.  What color does the pH indicator become?  Is this strong acid an electrolyte?  Does current travel through this solution?

·         Repeat the above tests with the weak acid, the strong base, and the weak base, and water.  Collect your observations in the table below:

 

	Strong Acid	Weak Acid	Strong Base	Weak Base	Water
pH meter read (value)	2.00	4.50	12.00	9.50	7.00
pH paper (color)	2 red	4 orange	11 blue	9 green	7 yellow
Conductivity (bright/dim/none)	bright	dim	bright	dim	none
Exists as Mostly (ions/molecules)	ions	ions	molecules	molecules	ions

Procedure:

This simulation allows you to change the concentration of a strong and weak acid and base.

Complete the table below for some strong acids and bases and weak acids and bases by adjusting the concentration.

Strong Acids

Strength	Initial Acid Concentration (mol/L)	[HA] (mol/L)	[A-] (mol/L)	[H+] (mol/L)	pH
	.010 M	Negligible	1.00 x 10^-2	55.6	2
	.050 M	Negligible	5.00 x 10^-2	55.6	1.30
	.100 M	Negligible	1.00 x10^-1	55.5	1
	1.00 M	Negligible	1.00 x10^0	54.6	0

Weak Acids

Strength (approximately)	Initial Acid Concentration (mol/L)	[HA] (mol/L)	[A-] (mol/L)	[H+] (mol/L)	pH
	.015 M	1.50 x 10^-2	4.20 x10^-5	55.6	4.38
	.150 M	1.50 x 10^-1	1.33 x10^-4	55.6	3.88
	.015 M	9.61 x10^-4	1.40 x10^-2	55.6	1.85
	.150 M	4.94 x10^-2	1.01 x10^-1	55.5	1.00

Strong Bases

Strength	Initial Acid Concentration (mol/L)	[MOH] (mol/L)	[M+] (mol/L)	[OH-] (mol/L)	pH
	.010 M	Negligible	1.00 x 10^-2	1.00 x 10^-2	12.00
	.050 M	Negligible	5.00 x 10^-2	5.00 x 10^-2	12.70
	.100 M	Negligible	1.00 x10^-1	1.00 x10^-1	13.00
	1.00 M	Negligible	1.00 x10^0	1.00 x10^0	14.00

Weak Bases

Strength (approximately)	Initial Acid Concentration (mol/L)	[B] (mol/L)	[BH+] (mol/L)	[OH-] (mol/L)	pH
	.015 M	1.50 x 10^-2	3.46 x 10^-5	3.46 x 10^-5	9.54
	.150 M	1.50 x 10^-1	1.10 x 10^-4	1.10 x 10^-4	10.04
	.015 M	2.33 x 10^-4	1.48 x 10^-2	1.48 x 10^-2	12.17
	.150 M	1.85 x 10^-2	1.32 x 10^-1	1.32 x 10^-1	13.12

Conclusion Questions:

1.      A strong acid is very concentrated / exists primarily as ions. (circle)

2.      A weak base is a nonelectrolyte / weak electrolyte / strong electrolyte.

3.      A strong base is a nonelectrolyte / weak electrolyte / strong electrolyte.

4.      At the same concentration (Molarity) a strong acid will have a higher / lower / the same pH as a weak acid.

5.      As concentration of a weak acid increases, the pH increases / decreases / remains constant.

6.      As concentration of a weak base increases, the pH increases / decreases / remains constant.

7.      As the concentration of a weak acid increases, the number of ions increases / decreases / remains constant.

8.      As the concentration of a weak acid increases, conductivity increases / decreases / remains constant.

9.      As the strength of a weak acid increases, the proportion of ions to molecules increases / decreases.

10.  As the strength of a weak acid increases, the conductivity increases / decreases / remains constant.

11.  What are the pH values of a weak acid with a concentration of 0.10 and a strong acid with a concentration of 0.01, ten times lower?     Weak acid, 0.10 M :___4.0_________           Strong Acid, 0.01 M :____2.0________