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18 Dec 2019

A variety of other procedural errors can impact the effectiveness of the copper reaction cycle. Comment on the following documented errors:

a) When NaOH (aq) is added to the Cu(NO3)2 solution in Reaction B, the Cu(OH)2 (s) occasionally does not precipitate immediately, and excess NaOH must be added. What other substance present in the reaction mixture might preferentially react with the NaOH and prevent precipitation of the Cu(OH)2? Explain .

b) Students will occasionally use HNO3 (aq) instead of H2SO4 (aq) in Reaction E, Step 5, assuming that both strong acids will accomplish the same purpose. Briefly describe the consequences of this error.

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Hubert KochLv2

31 Dec 2019

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I need help writing a purpose for a chemistry experiment. I feel I was always do a great job, but my professor always gives me 3 out of 6. Iâm out of ideas. She wants us to summarize the introduction, and summarize what you will do in each section of the experiment and state how this work will support the goal of this experiment being longer than 1/2 a page. She also said not to go into depth on calculations just summarize each section in procedure. Please, please help me right a better purpose for the experiment below. Thanks.

Experiment 5 Analysis of Copper in a Brass Sample

Introduction

A.General

The purpose of this experiment is to determine the percentage of copper present in an unknown brass sample spectrophotometrically using a Spectronic 20. Brass is an alloy consisting of tin, lead, copper, and zinc. The brass sample has already been ground to a fine powder.

A standard curve (graph) will be made for Cu⁺² in which a plot of absorbance (instrument reading from the Spectronic 20) versus molarity of Cu⁺² for various solutions will be made. These standard Cu⁺² will be made by dissolving the appropriate amount of CuSO₄ * 5H₂O is distilled water. The brass samples will be made by dissolving the brass in concentrated HNO₃ to produce Pb⁺², Cu⁺², Zn⁺² is solution along with the finely divided white hydrated tin (IV) oxide solid. After the properly prepared solutions have been filtered to remove the tin (IV) oxide, the absorbance of these solutions will be measured using the Spectronic 20. The molarity of Cu⁺² in these solutions will be obtained from the standard curve. Then the percent copper in the brass sample will be calculated.

B.Method of Analysis

In each of the 5 solutions (3 for determining the standard curve and 2 for the determination of copper in brass) the molarity of Cu⁺² present will be determined spectrophotometrically using the Bausch & Lomb Spectronic 20. The only species in the solution that absorbs at 620 mÂµ (this wavelength corresponds to the visible portion of the electromagnetic waves) in Cu⁺². In this aqueous solution Cu⁺² is really present as the aquo complex [Cu(H₂O)₆]⁺². The other species Pb⁺², Zn⁺², H⁺, NO₃^-, SO₄^-2, and H₂O do not absorb at the 620 mÂµ wavelength of light (and tin is precipitated as hydrated tin (IV) oxide).

In the Spectronic 20, light of wavelength of 620 mÂµ and certain initial intensity, I₀ is allowed to pass through the sample. The wider the sample tube (width = b) and the greater the molarity of Cu⁺², the more absorption will occur causing the intensity of the 620 mÂµ wavelength (I) to be less as it leaves the solution. The following equation describes the process quantitatively: A = log I₀ / I = abM, where A = absorbance (quantity actually measured by the Spectronic 20), M = molarity of the absorbance species (Cu⁺² in this experiment), b = tube diameter, and a = constant which is characteristic of each absorbing species. Since the same tube (a special tube called a cuvette is used for spectrophotometry) is used throughout the experiment, b remains constant. Hence, we can define a new constant, K = ab. Therefore, the above equation becomes: A = KM = K [Cu⁺²]. A plot of A (the absorbance value measured by the use of the Spectronic 20) versus the molarity of Cu⁺² should be a straight line going through the origin (A = 0.000 and M = 0.000).

Three standard Cu⁺² solutions will be made by dissolving a known amount of CuSO₄ * 5H₂O in distilled water. Hence, the molarity of these solutions will be known. Then the absorbance value for each of these three solutions will be measured using the Spectronic 20. A plot of the three absorbance values versus the corresponding Cu⁺² molarities will be done. The best straight line is drawn through these three points and the origin.

With the standard curve, the molarity of Cu⁺² in any Cu⁺² solution can be determined by measuring the absorbance value of the solution (always using the same cuvette and Spectronic 20) and referring to the standard curve to find the molarity of Cu⁺² that corresponds to that absorbance value. Thus, the molarity of Cu⁺² in the brass sample solutions can be obtained from the standard curve after their absorbance values have been measured on the Spectronic 20.

Procedure

Preparation of Brass Sample Solutions

Weigh two brass samples between 0.6 to 0.8 g (to three d.p.). This is done by putting a small clean, dry beaker (100 or 150 mL) on the pan of the balance. Zero the balance. Carefully add the unknown to the beaker until you get a weight somewhere between 0.6 to 0.8 g (Record to 3 d.p.). To each beaker (under the hood) add 12 mL of 8 M nitric acid very slowly. The reaction of 8 M acid with the finely divided brass sample occurs vigorously. The 8 M must be added slowly to prevent splattering of the brass sample out of the container. The reaction is performed in the hood to prevent toxic brown NO₂ gas from entering the lab. After the initial reaction, leave the beaker in the hood but mix occasionally over 30 mins (during this 30 mins the known CuSO₄ * 5H₂O solutions (Part B) can be measured on the Spectronic 20.)

After the brass has reacted with the 8 M nitric acid, wash down the sides of the beaker with some distilled water from a squeeze bottle. Quantitatively, with no spillage transfer the mixture to a clean 50 mL volumetric flask. After solution is poured into the volumetric flask, add more distilled water to the beaker, washing the sides of the walls. Pour this quantitatively into the volumetric flask as mentioned above. Repeat this two more times. Then add enough distilled water until the 50.00 mL mark is reached by the bottom of the meniscus of the solution. Stopper and mix very well by continually shaking the flask in the upright and inverted positions.

The blue solutions must be filtered to remove the tin (IV) oxide or an incorrect high reading will be obtained for the absorbance reading due to the scatter of the light waves. Filter each of the solutions through dry Whatman No. 1 filter paper. (Use 2 pieces to avoid another filtration. Place one piece on the other. Fold in half and then fold in half again. Open it up into a cone and place it into a filter funnel.) Collect the filtrate in clean, dry beakers. If the filtrate is not a clear pale(blue) color, then another filter again with new Whatman filter paper and with clean, dry beakers. One to three filtrations maybe needed, depending on how much tin was present in the brass sample. Now measure the absorbance of these two solutions as described in the section âUse of the Spectronic 20.â Be sure you do these absorbance measurements and the three from part B on the same Spectronic 20.

Preparation of known CuSO₄ * 5H₂O solutions

The three known (standard solutions have already been prepared for you. They are ready for use in their cuvettes by the side of the Spectronic 20 instrument. These three solutions were prepared for you by the stockroom personnel as described in the procedure in the next paragraph. All you need to do is to measure the absorbance of these three prepared standard solutions.

Weigh three 50 mL volumetric flasks (to 3 d.p.). To each of these volumetric flasks add about 1.000, 2.000, and 3.000 grams of CuSO₄ * 5H₂O. Again weigh the volumetric with the CuSO4 * 5H2O (to 3 d.p.). Add 6.0 mL of concentrated nitric acid (measured with a graduated cylinder) and approx. 25 mL of distilled water to each flask. Mix until CuSO₄ * 5H₂O dissolves. Then add distilled water until the bottom of the meniscus of the solution hits the 50.00ML mark. Stop and mix well (as previous section for making the brass solutions). Measure the absorbance of each solution as described in the section âUse of the Spectronic 20.â Be sure you do these absorbance measurements and the three from part A on the same Spectronic 20.

Use of Spectronic 20

How to use the Spectronic 20 and measure the absorbance of each solution is described on another document I will give. Remember wavelength should be set at 620 mÂµ.

Having trouble with thse questions. Please answer all questions correctly. Will provide a thumbs up.

1.Which of these equations is the balanced reaction between nitrous acid solution and solidmagnesium hydroxide ?

A.2NH₃(aq) + MgOH (s) Mg(NH₄)₂(aq) + 2 H₂O(l)

B.2HNO₂(aq) + Mg(OH)₂(s) Mg(NO₂)₂(aq) + 2 H₂O(l)

C.2HNO₃(aq) + Mg(OH)₂(s) Mg(NO₃)₂(aq) + 2 H₂O(l)

D.2HNO₂(aq) + MgOH (s) Mg(NO₃)₂(s) + 2H₂O(l)

2.Balance the following chemical equation:

C₃H₇OH(l) + O₂(g)CO₂(g) + H₂O(g)

a. C₃H₇OH(l) + 4 O₂(g)3 CO₂(g) + 4 H₂O(g)

b.2 C₃H₇OH(l) + 9 O₂(g)6 CO₂(g) + 8 H₂O(g)

c.C₃H₇OH(l) + 5 O₂(g)3 CO₂(g) + 4 H₂O(g)

d.C₃H₇OH(l) + O₂(g)CO₂(g) + H₂O(g)

3.Which species would be considered a spectator ion based on the following equation?

Ba²⁺ + 2NO₃^â + 2Na⁺ + SO₄^2- â BaSO₄(s) + 2NO₃^â + 2Na⁺

SO₄^2-

2NO₃^â

Ba²⁺

BaSO₄

4.Barium sulfate is often ingested to aid in the diagnosis of GI tract disorders. The equation for its production is Ba²⁺ + SO₄^2- â BaSO₄(s). What type of equation is this?

A full equation

A net ionic equation

A total ionic equation

An oxidation-reduction equation

5.Which species is being reduced in this reaction?

CuO(s) + H₂(g) Cu(s) + H₂O(g)

H₂O

CuO

H₂

6.Which of the following is an oxidizing agent?

H₂

NH₃

O₂

H₂O

7.The following chemical equation,

HC₂H₃O₂(aq) + KOH(aq) KC₂H₃O₂(aq) + H₂O(l),

is an example of a(n) _______ reaction.

a.oxidation-reduction and acid-base

b.acid-base

c.precipitation

d.oxidation-reduction and precipitation

e.oxidation-reduction

8.What substance is oxidized in the following reaction?

4HCl + MnO₂ â Cl₂ + 2H₂O + MnCl₂

a.Mn in MnO₂

b.O in MnO₂

c.Cl in HCl

d.H in HCl

9.What is the reducing agent in the following reaction?

4HCl + MnO₂ â Cl₂ + 2H₂O + MnCl₂

Cl₂

MnCl₂

HCl

MnO₂

10.All redox reactions involve oxygen.

True

False

11.The term formula weight can be used with respect to which of the following?

A.ionic compounds

B.covalent compounds

C.both a and b

D.neither a nor b

12.What is the molecular weight of ibuprofen, C₁₃H₁₈O₂?

A.206.3 amu

B.377.7 amu

C.29.0 amu

D.289.4 amu

13.The mass of an atom of ¹²C is exactly 12 atomic mass units. Which of the following describes a mole of ¹²C atoms?

A.a sample which weighs exactly 12 atomic mass units

B.a sample which contains 6.02 Ã 10²³ atoms of ¹²C

C.both a and b

D.neither a nor b

14.The mass of an atom of ¹²C is exactly 12 atomic mass units. Which of the following describes a mole of ¹²C atoms?

A.a sample which contains 6.02 Ã 10²³ atoms of ¹²C

B.a sample which weighs exactly 12.011 grams

C.both a and b

D.neither a nor b

15.What is the molar mass of water, H₂O?

A.18.02 g

B.18.02 amu

C.17.01 g

D.17.01 amu

16.What is the molar mass of glucose, C₆H₁₂O₆?

A.180.2 amu

B.180.2 g

C.60.05 amu

D.60.05 g

17.In the reaction 2H₂O â 2H₂ + O₂, 2.0 mol water will produce how many grams of O₂?

18.In the reaction 2CH₄ + O₂ â 2CO + 4H₂, how many molecules of CO will be produced by the complete reaction of 1.60 grams of CH₄?

3.01 Ã 10²²

2.00

12.0 Ã 10²³

6.02 Ã 10²²

19.Suppose 2.43 g of magnesium is reacted with 3.20 g of oxygen: 2Mg + O₂ â 2MgO. How many grams of MgO will be produced?

1.27

4.03

5.63

8.06

20.Which of the following conversion factors would allow one to calculate the amount of PbI₂produced from a given amount of NaI in the following unbalanced reaction?

Pb(NO₃)₂ + Nal â Pbl₂ + NaNO₃

21.The unbalanced equation for the reduction of copper(I) sulfide with oxygen is given below. What mass of copper is produced from the reduction of 7.50 g Cu₂S?

Cu₂S(s) + O₂(g) Cu(s) + SO₂(g)

18.8 g

5.99 g

1.50 g

9.39 g

2.99 g

22.How many grams of aluminum oxide are produced by the complete reaction of 19.8 g aluminum with excess oxygen?

10.5 g aluminum oxide

37.4 g aluminum oxide

49.9 g aluminum oxide

74.8 g aluminum oxide

23.If 5 grams of CO and 5 grams of O₂ are combined according to the reaction, which is the limiting reagent?

2CO + O₂ â 2CO₂

O₂

CO and O₂

CO₂

24.In an industrial process, nitrogen is reacted with hydrogen to produce ammonia. The percent yield of this particular process is 26%. How much hydrogen is needed to produce 4.8 kg ammonia?

1.5 kg

7.0 kg

0.85 kg

3.3 kg

25.A percentage yield greater than 100% has been reported in a scientific paper you are assigned to read. What would be a reasonable explanation for this?

a. The product was measured more than once.

b. It must be a typographical error since yields can't exceed 100%.

c. A lack of training of the professor who published the study.

d. Small measurement errors could have caused this to occur.

26.In an industrial process, nitrogen is reacted with hydrogen to produce ammonia. The percent yield of this particular process is 24.8%. How much hydrogen is needed to produce 3.7 kg ammonia?

1.8 kg

1.2 kg

4.4 kg

12 kg

2.7 kg

27.The following reaction has a percent yield of 90.5%. How many grams of carbon dioxide are produced from 78.5 g calcium carbonate via this reaction?

2 HNO₃(aq) + CaCO₃(aq) Ca(NO₃)₂(aq) + H₂O(l) + CO₂(g)

34.5 g

21.6 g

38.1 g

31.2 g

28.When gas molecules dissolve in liquids to form a saturated system, the process is usually

exothermic.

endothermic.

independent of the nature of the liquid.

temperature independent.

29.The reaction below would be considered.

CaCO₃ + heat â CaO + CO₂

an exothermic combination.