Southeast Community College Relationships in Chemical Substances Questions

Experiment 4Relationships in Chemical Substances
Objectives
•
•
•
To discover the relationship between reactants in a chemical reaction.
To react magnesium and hydrochloric acid together and recover the entire product formed.
To develop graphing skills and understand how slope can identify the empirical formula of a
molecule.
Chemical Hazards
3.5 mol/L (3.5 M) hydrochloric acid, HCl, is a strong acid. As a solution HCl is a corrosive substance
and an irritant to skin and eyes.
•
•
Skin contact can be detected by itching and/or redness. If you spill HCl on yourself, immediately
flush the exposed skin area with running water in the sink or, flush the eyes in the eyewash
station.
Hydrogen chloride fumes, which are produced when the solution is heated, are a respiratory
irritant. To avoid inhalation of the fumes, one part of the lab will be conducted in the chemical
fume hood, which vents to the outside.
Hydrogen gas is flammable and no open flames will be permitted during that part of the experiment.
Introduction
Many metals react with acids to form hydrogen gas and a soluble ionic compound. For example, zinc
and hydrochloric acid react as follows:
Zn(s) + 2 HCl(aq) → H2(g) + ZnCl2(aq)
Magnesium reacts in a similar albeit more exothermic manner (produces heat). You will react a sample
of magnesium turnings with HCl. Next, recover the product by removing the excess water through
evaporation. The solid product that remains is your compound. In the example above after evaporating
the water in the solution the remaining solid product would be zinc chloride.
Since you are using magnesium instead of zinc for this experiment, your solid will be magnesium
chloride.
HCl will be added in excess. Excess means you will be adding more HCl than you need in order to
completely react all of the magnesium. Therefore, the mass of magnesium in your final product will be
the same as the mass of magnesium you weigh out in the beginning. Assuming the product contains
only magnesium and chlorine, you can determine the mass of chlorine by subtracting the mass of
magnesium from the mass of your final product. The calculation section will walk through an example
which assumes a reaction between zinc and HCl.
Revised 6/7/2022
4.1
A mathematical relationship exists between the mass of metal and the mass of nonmetal in an ionic
compound. A mathematical relationship also exists between the moles of metal and the moles of the
nonmetal in an ionic compound. You will examine both relationships by making graphs of the data
collected by your lab section. The best-fit lines from the graphs will help determine which relationship is
the most appropriate when trying to write an empirical chemical formula: mass or moles?
Calculations
Example: You react 1.6765 g zinc with 25.00 mL 3.5 M HCl. At the end of the experiment 3.5870 g
product are recovered according to the chemical equation given previously:
Zn(s) + 2 HCl(aq) → H2(g) + ZnCl2(aq)
Since the mass of zinc is assumed to be the same as in the beginning and in your final product, the
mass of chlorine in your product can be calculated:
3.5870 𝑔 − 1.6765 𝑔 = 1.9105 𝑔
Using 65.38 g/mol (the molar mass of zinc from the periodic table), the moles of zinc used can be
calculated from the mass of zinc:
1.6765 𝑔
1 𝑚𝑜𝑙
×
= 0.02564 𝑚𝑜𝑙
1
65.38 𝑔
Using 35.45 g/mol (the molar mass of chlorine from the periodic table), the moles of chlorine can be
calculated from the mass of chlorine:
1.9105 𝑔
1 𝑚𝑜𝑙
×
= 0.05389 𝑚𝑜𝑙
1
35.45 𝑔
4.2
Procedure
Before beginning the procedure, read the steps 1-6 and pay special attention to step 4.
1. Clean and dry a 125 mL Erlenmeyer flask. Determine mass of the Erlenmeyer flask and record on
your data sheet. The mass of the empty flask is needed because you will recover your product in it
at the end of the experiment.
2. Each student will use a different assigned mass of magnesium. Once you know the mass of
magnesium, go to the balance and measure it on a weighing dish (remember to tare the weighing
dish before weighing the magnesium). You must be within ±0.0200 g of the assigned mass.
Transfer all of the magnesium to the Erlenmeyer flask weighed earlier.
3. Use a 25 mL graduated cylinder to measure 25.00 mL of a 3.5 M HCl solution. To minimize spilling
first pour approximately 25 mL to 30 mL into a 50 mL or 100 mL beaker. Next, carefully use the
spout part of the beaker to pour a little more than 25 mL of acid into your graduated cylinder. Adjust
the final volume to 25.00 mL using a dropping pipet.
4. Slowly add the 3.5 M HCl to the 125 mL Erlenmeyer flask containing the magnesium. Be careful to
only add a little at a time while swirling the flask. Wait until the bubbling slows before adding more
acid. The reaction between the two is vigorous and produces heat. Do not put your head directly
above flask’s opening because hydrogen gas is produced along with some HCl fumes. The reaction
should take between 5 and 10 minutes to complete and all of the magnesium must be reacted
before you can continue to the next step.
5. At this point your solution contains magnesium and chloride ions, excess hydrochloric acid, and
water. Since heating a solution containing hydrochloric acid causes the release of hydrogen
chloride gas, the solution must be heated in the hood using a hot plate that will drive off most of the
excess acid. Put your initials on your flask and place your flask on one of the hot plates set up in the
hood:
6. When your solution has been evaporated to total dryness, the instructor will remove your flask from
the hot plate and allow it to cool until approximately room temperature. Once the instructor gives
you your flask, record mass of the flask and product.
4.3
4.4
Name _________________________
Date __________
Section __________
Experiment 4
Relationships in Chemical Reactions
1. Data:
1a. Volume of 3.5 M HCl
_______________
1b. Mass of 125 mL Erlenmeyer Flask
_______________
1c. Mass of Magnesium Actually Used
_______________
1d. Mass of Product and Erlenmeyer Flask
_______________
2. Calculations:
2a. Calculate the mass of product in the Erlenmeyer flask:
2b. Calculate the mass of chlorine in the product:
2c. Calculate the amount of magnesium in the product in moles:
2d. Calculate the amount of chlorine in the product in moles:
Revised 12/17/2021
4.5
3. Class’s Data:
Assigned
Actual Mass
mass of Mg
of Mg
(g)
(g)
Mass of
Product
(g)
Mass of Cl
(g)
Amount of Mg
(mol)
Amount of Cl
(mol)
Examine the information in the Class’s Data Table and write two to three sentences about your
observations between different variables (mass of magnesium, mass of chlorine, etc.).
4.6
Outlier?
Analysis
1. Use information from the data table to make a graph (on paper) of the mass of chlorine as a function
of the (actual) mass of magnesium. Do not include the outliers, if any, which were identified by your lab
instructor in the graph and do not use them to determine the best-fit line. Include your graph with your
report.
1a. Use the Chemistry Lab Resources website and determine the slope and the y-intercept of your
graph and then draw the best representation of the best-fit line on your graph.
Slope (3 decimals): _______________
Y-Intercept (3 decimals): _______________
1b. If the data points appear to lie on a relatively straight line that runs through or close to the origin,
then the data is said to be represented by a direct proportion. Is the mass of chlorine in direct
proportion to the mass of magnesium? Yes No
1c. Interpolation is an estimation between variables within the range of a data set while extrapolation is
an estimation that falls outside the range of a data set. Using only the graph, first write down whether
each is an interpolation or an extrapolation. Then if it’s an interpolation, answer the question. If it’s an
extrapolation, don’t answer the question. You can use the graph and estimate the mass of magnesium
or chlorine based on the given mass of chlorine or magnesium.
(i) What mass of magnesium will be needed to give 2.04 g chlorine?
(ii) What mass of chlorine will be produced if 5.75 g magnesium reacts?
(iii) When 0.51 g magnesium reacts, what mass of chlorine will be produced?
1d. The slope from 1a is the ratio of the mass of chlorine to the mass of magnesium in the product.
Take that value for the slope and round it to the nearest whole number.
1e. Calculate the percent error of the slope with respect to the nearest whole number. The slope from
1a is the experimental value, EV, and the rounded whole number from 1d as the standard value, SV.
For significant figure purposes assume the SV is an exact number.
𝐸𝑉 – 𝑆𝑉
× 100 = % error
𝑆𝑉
4.7
2. Use information from the data table to make a graph (on paper) of the moles of chlorine as a function
of the moles of magnesium. Do not include the outliers, if any, which were identified by your lab
instructor in the graph and do not use them to determine the best-fit line. Include your graph with your
report.
2a. Use the Chemistry Lab Resources website and determine the slope and the y-intercept of your
graph and then draw the best representation of the best-fit line on your graph.
Slope (3 decimals): _______________
Y-Intercept (3 decimals): _______________
2b. If the data points appear to lie on a relatively straight line that runs through or close to the origin,
then the data is said to be represented by a direct proportion. Is the moles of chlorine in direct
proportion to the moles of magnesium? Yes No
2c. Interpolation is an estimation between variables within the range of a data set while extrapolation is
an estimation that falls outside the range of a data set. Using only the graph, first write down whether
each is an interpolation or an extrapolation. Then if it’s an interpolation, answer the question. If it’s an
extrapolation, don’t answer the question. You can use the graph and estimate the moles of magnesium
or chlorine based on the given moles of chlorine or magnesium.
(i) What amount of magnesium in moles will be needed to give 0.050 moles of chlorine?
(ii) What amount of chlorine in moles will be produced if 0.010 moles of magnesium reacts?
(iii) When 0.061 moles of magnesium reacts, what amount of chlorine in moles will be
produced?
2d. The slope from 2a is the ratio of the mass of chlorine to the mass of magnesium in the product.
Take that value for the slope and round it to the nearest whole number.
2e. Calculate the percent error of the slope with respect to the nearest whole number. The slope from
2a is the experimental value, EV, and the rounded whole number from 2d as the standard value, SV.
For significant figure purposes assume the SV is an exact number.
𝐸𝑉 – 𝑆𝑉
× 100 = % error
𝑆𝑉
4.8
3. You prepared two graphs earlier, one graph of the mass of chlorine as a function of the mass of
magnesium (question #1) and the other of the amount of chlorine in moles as a function of the amount
of magnesium in moles (question #2). Which graph do you believe is the most appropriate one to use in
order to determine the chemical formula of magnesium chloride experimentally? Clearly explain the
reason(s) for your choice.
4. Based on the graph you chose in question #3, what is the experimental chemical formula for
magnesium chloride? Clearly explain your answer by using the slope of the best-fit line.
5. Using only the periodic table, what is the expected chemical formula for magnesium chloride? Clearly
explain by using relative charges based on the elements’ locations in the periodic table.
6. Do your experimental results from question #4 match the expected results in question #5? If they do
not match then write a few sentences to suggest a reason as to why they don’t match.
4.9