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A 12.63-mL sample of aqueousNH3 was reacted with 50.00-mL of 0.1025 M HCl. Theexcess acid is then neutralized by titrating the sample to aphenolphthalein end-point with 25.98-mL of 0.1445 M NaOH. Thismethod is known as an indirect method of analysis.

a. Explain why the reaction between NaOH and HClallows you to determine the molarity of the NH3solution.

b. Show the series of calculations required tocalculate the molarity of NH3 in the aqueoussolution.

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Related questions

1) You weigh an empty gas buret, fill it to the 50.00 mL mark withdistilled water and weigh it again.
Experimental Data

Mass of empty gas buret 43.22 g
Mass of gas buret filled to 50 mL mark with water 96.89 g
Temperature of the water 28.7 °C
Density of water... @ 20.0 °C 0.9982 g/cc; D35 °C=0.9941 g/cc
Specific Gravity of Mercury... 13.55
Calculate:

a) Density of water at 28.7 °C (by interpolation) ______________g/cc
b) Volume of water filling buret to 50.00 mL mark ______________mL
c) Extra volume of water in addition to the 50.00 mL ______________mL
2) You are asked to make 128 mL of 1.199 M HCl starting with 9.0 MHCl.

d) The volume of 9.0 M HCl needed is (to 0.1 mL) ______________mL
3) You standardize the "1.199 M" HCl by pipetting a known amount ofit into a flask and titrating to the phenolphthalein endpoint withan NaOH solution of known concentration
Experimental Data:

Volume of "1.199 M" HCl used 4.06 mL
Concentration of NaOH used 0.1714 M
Initial buret reading 0.00 mL
Buret Reading at endpoint 28.69 mL
Calculate:

e) Exact molarity of your HCl ______________ M
4) You pipet a known amount of this standardized HCl into a 15 cmsample tube and float a "boat" containing your unknown.
You hook up the sample tube and contents to the gas buret.
You shake the tube to start the reaction.
When the metal unknown is completely dissolved, you read the gasburet and measure the height of the buret's meniscus above thewater level.
Experimental data:

Volume of standardized HCl used 4.04 mL
Weight of metal unknown used 34.9 mg
Gas buret reading after reaction is compete 26.62 mL
Height of meniscus above water level 11.9 cm
Atmospheric pressure 752.4 mmHg
Temperature of the water 23.3 °C
Calculate:

f) Vapor pressure of water at 23.3 °C
interpolate data at... http://genchem.rutgers.edu/vpwater.html______________ mmHg
g) Total volume of gas released ______________ mL
h) Partial pressure of hydrogen ______________ mmHg
i) Equivalent weight of metal
based on amount of hydrogen generated ______________ g/eq
5) The remaining solution in the 15 cm sample tube containing thereaction products and the unused HCl is quantitatively transferedto a flask and titrated to its endpoint with the standardNaOH.
Experimental data:

Initial volume of buret 0.16 mL
Final volume of buret 13.17 mL
Calculate:

j) Equivalent weight based on the amount of acid used up______________

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. I’m 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 mµ (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 mµ wavelength of light (and tin is precipitated as hydrated tin (IV) oxide).

In the Spectronic 20, light of wavelength of 620 mµ 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 mµ 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 “Use of the Spectronic 20.” 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 “Use of the Spectronic 20.” 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 mµ.

This is for the lab practical I take on Tuesday night. If you could answer each one and show your work/ explain that'd be amazing so I can do well on the actual practical on Tuesday! Thank you!

Chemistry 211 Lab Final Study Guide Spring 2015

Useful Information:
1 ampere =coulomb/sec
Faraday’s constant = 96,500 coulombs/mole of electrons
Molar mass of Au = 197.0 g/mole, Molar mass of Cu = 63.55 g/mole
m = moles , ' ’

c kg

ΔTf

= Tf

− Tf

where Tf is for the pure solvent and Tf

is for the mixture.

ΔT = (K

)(m ) ,

m = (

(mass of

solute, g )
)( ).

f f c

c molar

mass, g

mol −1

mass of

solvent, kg


7. Next question

(a) If you weigh 6.355 grams of copper, add concentrated nitric acid to dissolve it, and then dilute the solution to a total volume of 50 mL, how many moles of copper are in this solution?


(b) For the equilibrium reaction 3H2(g) + N2(g) → 2NH3(g), what is the equilibrium constant expression, Kc, for this reaction? (5pts)

8. Suppose at some temperature, the equilibrium concentration of H2 is 0.45M, the equilibrium concentration of N2 is 0.38M, and the equilibrium concentration of NH3 is 0.42M. What is the value of the equilibrium constant for the reaction 3H2(g) + N2(g) → 2NH3(g) at this temperature? (Show your work. No credit will be given if your work is not shown.)

9. Next question
(a) If you need 0.0534 moles of NaOH to reach equivalent point in the titration with acetic acid, how many mL of 0.09876M NaOH will you need?


(b) The Ksp of Cu(OH)2 is 2.2x10-20, what is the solubility of Cu(OH)2?


10. Next question
(a) In non-sulfide qualitative analysis of representative cations, the Group A cations comprising of Pb2+,

Hg2+, and Ag+ give a precipitate with solution.
(b) From a solution containing mixture of Pb2+, Fe3+, and Al3+, Pb2+ is removed by adding HCl solution. To the remaining cation mixture, on making the solution basic with NH3 (pH=9 – 10), both the cations form precipitate. Further analysis of the cation mixture containing Fe3+ and Al3+ is carried out by adding NaOH solution.


Aluminum hydroxide (dissolves/remains undissolved) in NaOH solution while Ferric hydroxide ( dissolves/remains undissolved).
(c) Fe(OH)3 precipitate is dissolved in minimal amount of aqueous HCl which gives a


yellow solution. To this solution addition of solution (at pH = 4 – 5) yields a dark blue solid known as Prussian Blue.

(d) Describe the method for checking for completeness of precipitate in qualitative analysis of ions.

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