# ACCHS Acid and Bases Chemistry lab

Now that you’ve had some practice with acid/base chemistry and the cell phone spectrometers,it’s time for you to think about experimental design. Our system of study will be buffers.
Remember that buffers are made by either combining a weak acid with its conjugate base or a
weak base and its conjugate acid. A buffer is able to withstand significant changes to pH upon
addition of an acid or base. The target pH of a buffer should be within +/- 1 pH unit of the
buffer’s pka.
Challenge Goal: Your goal is to demonstrate that for an unbuffered solution vs a buffered
solution, the buffered solution resists drastic changes to pH when acid or base are added.
As you design the experiment, take careful notes and include all reasoning and consideration.
We will be writing a formal lab report for this lab. Questions for consideration are listed below.
Important information
1. Our buffer system: NaHCO3 (baking soda) and Na2CO: (washing soda).
Question: What is the acid and what is the conjugate base? Write the chemical
equation for the weak acid dissociation in water.
Question: What is the pka of the weak acid dissociation?
2. Washing soda (Na2CO3) can be purchased or made by heating baking soda on a tray in the
oven at 200 deg C (400 deg F) for ~60 minutes.
3. Pick a target pH for your buffer“ remember that depends on the weak acid pka. Use the
Henderson-Hasselbalch equation to determine what ratio and quantities of conjugate base and
weak acid you need.
What is your final target volume?
What molarities of NaHCO3 and Na2CO3 do you need?
How can you measure amounts of NaHCO3 and Na2CO3 using teaspoons? How can
you calculate moles from that information?
.
Question: Show your work for calculating how much water, NaHCO3, and Na2CO3 are
needed to make the buffer.
4. Useful measuring conversions:
1/8 tsp
0.6 mL
1.25 ml
2.5 mL
9/4 tsp
3.75 mL
1 tsp
5 mL
Atsp
2 tsp
5. How will you measure pH? Record observations (color) and measure pH.
1
6. Your experiment should include the following tests:
What is the pH of the nonbuffered solution, buffered solution?
Test the pH of the buffer, does it match your target pH? What is the initial pH of
the nonbuffered solution?
How does the addition of acid affect the nonbuffered solution? What about base?
How much acid or base must be added to the buffer to get it to the same pH as the
nonbuffered solution?
.
3:26
..1 LTE O
< Back No Comments Important Information 1. Our buffer system: NaHCO3 and Na2CO3 o Question: What is the acid and what is the conjugate base? Write the chemical equation for the weak acid dissociation in water. o Answer: 0 Na2CO3 ---> 2Na+ + CO32- and NaHCO3 —> Na+ +
HCO3- o HCO3-(aq) + H2O(1) = CO3-2(aq) + H30+
(aq) o Hence, HCO3- is the weak acid and the
CO3-2is the conjugate base. 2. Washing soda can be
purchased or made by heating baking soda on a tray in
the oven at 200 deg C for approximately 60mins. 3.
Pick a target pH for your buffer, remember that
depends on the weak acid pka. Use the Henderson-
Hasslebalch equation to determine what ratio and
quantities of conjugate base and weak acid you need.
o The Ka2 of HCO3-(aq) is equal to 4.7×10-11
(literature value) o With this reaction, HCO3-(aq)+
H2O(1) = CO3-2(aq) + H3O+(aq) we can compute the
ratio using Henderson-Hasslebalch equation – The
Henderson-Hasslebalch equation is, pH = pka + log10
([base] / [acid]) • We’ll gonna use pH = 10.40 (nearest
to pKa value), the ratio computations are as follows, o
pH = pka + log10 ([base] / [acid]) o 10.40 = 10.33 +
log ( [Na2CO3] / [NaHCO3])[Na2CO3] /
[NaHCO3] = 1.2 o Question: How much water,
NaHCO3, and Na2CO3 are needed to make a buffer o
buffer in a 500mL volumetric flask, below is the
calculations mol HCO3- + mol CO32- = (0.200 mol/
L) x (0.500L) o mol HCO3- + mol CO32- = 0.100mol
3:26
..1 LTE O
< Back No Comments moles of Na2CO3, o 0.100 mol Na2CO3 x mol105.99g = 10.60g Na2CO3 - Next, we add HCl, converting 0.07426 moles of HCO3-(aq) and CO3-2(aq) as shown by the following reaction, o CO3-2(aq) + H30+(aq) > HCO3-(aq) + H2O(1) Thus, we need 0.07426 moles
of HCl, or o 0.07426 mol HCI x OmollL x 1L1000mL =
12.38mL HCI . To prepare the buffer we add 10.60 g
of Na2CO3 to a 500-ml volumetric flask and dissolve
it with some water. We then add 12.38 mL of 6 M HCI
and dilute to volume. Explanation: Explanations: 1.
HCO3- is a weak acid based on the Bronsted-Lowry
definition, an acid is an H+ (proton) donor. The CO32-
is the conjugate base formed. 2. Ka2 is the second
acid dissociation constant of NaHCO3 since its a
polyprotic acid. And to determine the Acidity (pka) of
HCO3-(aq) we need to compute the negative log of the
Ka2 which is equal to 10.33. 3. Since the experiment
suggest that you’re the one to decide the volume and
the concentration of your buffer well gonna assume the
values that will be used in the calculation. Given the
buffer ratio, we can project the needed weights of the
weak acid and conjugate base pair in its specified
volume and concentration. 4. For experimental reasons,
a buffer prepared following one of these approaches
probably will not produce a solution with a pH that
matches exactly the desired pH. When you prepare a
buffer in the laboratory it often is necessary to adjust
the buffer’s pH level to the desired value by adding
small amounts of either a strong acid or a strong base
while monitoring the pH with a pH electrode.
3:26
..1 LTE O
of HCl, or o 0.07426 mol HCI x OmollL x 1L1000mL =
12.38mL HCI . To prepare the buffer we add 10.60 g
of Na2CO3 to a 500-ml volumetric flask and dissolve
it with some water. We then add 12.38 mL of 6 M HCI
and dilute to volume. Explanation: Explanations: 1.
HCO3- is a weak acid based on the Bronsted-Lowry
definition, an acid is an H+ (proton) donor. The CO32-
is the conjugate base formed. 2. Ka2 is the second
acid dissociation constant of NaHCO3 since its a
polyprotic acid. And to determine the Acidity (pka) of
HCO3-(aq) we need to compute the negative log of the
Ka2 which is equal to 10.33. 3. Since the experiment
suggest that you’re the one to decide the volume and
the concentration of your buffer well gonna assume the
values that will be used in the calculation. Given the
buffer ratio, we can project the needed weights of the
weak acid and conjugate base pair in its specified
volume and concentration. 4. For experimental reasons,
a buffer prepared following one of these approaches
probably will not produce a solution with a pH that
matches exactly the desired pH. When you prepare a
buffer in the laboratory it often is necessary to adjust
the buffer’s pH level to the desired value by adding
small amounts of either a strong acid or a strong base
while monitoring the pH with a pH electrode.
12:46
X
Online – Acid/Base and Buffers Rubric
ООО
Acid/Base Chemistry Lab Rubric – Weel
Comment
Overall (10 points)
Consistent Verb Tense
Units are present
“Human error” is not used
Double Spaced
Grammar and Spelling are correct
Title (5 points)
Abstract (5 points)
4 – 5 sentences describing hypothesis, goal of experiment, and major
conclusions
Introduction (15 points)
Explain the definitions of acids and bases
Goals of experiment are given
Overview and purpose of each step and technique is given
– Use of cabbage indicator
– How is color measured?
• Titration – what is the goal? How does the experiment work?
• Buffers
Procedure (15 points)
Give a description of how you completed the entire lab sequence (weeks 7
– 9)
Data and Results (20 points)
Include data for each part in either words, table form, or graph. Do NOT
just copy/paste tables from Excel – prepare new tables in Word.
A description of Results in paragraph form – but refrain from analyzing
data
Discussion (20 points)
Overall Analysis of procedure – did all steps go as planned? What errors
were encountered?
What are key results from the titration? From the buffer experiment? How
can your results be explained in terms of what we are learning in class
References (5 points)
5 references minimum
Proper citation style (MLA or APA ok) and in-text citations are used
Tables and Figures (5 points)
Proper formatting of Tables including titles, tables are numbered, columns
Proper formatting of Figures including captions, figures are numbered,
graphs have labels for axes (including units)
12:46
X Formal Lab Report – General Instruct…
ООО
In the discussion section, you will analyze your data. Compare
values agree with theoretical values? If not, what happened in
lab during your experiment that might have led to poor
agreement. Do not be vague and do not use “human error.” This
phrase is meaningless and vague. In some cases, you will also
need to interpret your data – what are their implications?
– Explain what the data means include relevant numerical
values (like the key data points acquired).
– Draw conclusions based on experiment.
– Compare your values with theoretical values (if
available).
– Discuss sources of error and how they might have
wrong’ don’t qualify).
Citations & References
Example 1
You should always use in-text citations that are linked to an
proper
style guide. You can use MLA or APA (“Purdue Online Writing
Lab”, n.d.). Note that I just used in-text citations to cite the MLA
and APA styles. The complete entry for this source will appear
in the sample bibliography below. Please correctly cite websites.
Do not use quotations – the point of the lab report is for you to
show me that you understand the material.
Bibliography
https://owl.purdue.edu/owl/purdue owl.html
Example 2
There are other in-text citation styles that use superscript; the
style of the American Chemical Society is one such example.’
Bibliography
https://libguides.williams.edu/citing/acs.
Tables and Figures are encouraged, but please always give each
table or figure a number (example: Table 1 or Figure 1) and
a title for tables or caption for figures. The title or caption
is presented.

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