Hello everyone, I need help with these problems, I have a exam coming up within

Question

Hello everyone, I need help with these problems, I have a exam coming up within two days, so my message to you is that for some problems can you show your work for example for Q1 & Q3. Thank you, i would really appreciate it.

1. Suppose that you are using the AAS to determine the concentration of Ca in Minute Maid Orange Juice whose label states that its Ca content is 300 mg/240 ml serving. What sample amount would you take in order to prepare a 100 ml solution whose Ca concentration is 50 ppm (which is the middle of the AAS linear range for Ca). (Atomic weight of Ca = 40.08 g/mole)

A) 1.25 ml

B) 4.0 ml

C) 6.25 ml

D) 40 ml

E) none of the above

2. Which of the following statements about atomic absorption spectroscopy are true?

A) It measures light emitted by excited atoms as they return to the ground state .

B) There is no need for a monochromator in an AAS instrument.

C) There is no need for a light source in an AAS instrument.

D) The main purpose of the flame in an AAS instrument is to vaporize and atomize the sample.

Q3. How many equivalence point would you expect to obtain for complete titration of 25.00 ml of 0.10 M H3PO4 with 0.20 M NaOH? Calculate the equivalence point volume(s).

Explanation / Answer

Question 2.

Option A. It measures light emitted by excited atoms as they returno to the ground state. Basically it needs the use of a monochromator and a light source to do that, so Option B and C cannot be true. Option D is true, but that's not the only purpose for the flame, that's why I don't choose it as a true statement.

Question 3.

H3PO4 is a triprotic acid, so, I would expect as least 3 equivalence point. The acid would go from H3PO4 ----> H2PO4- -------> HPO42- --------> PO43-

So at least you need 3 equivalence point to do that. How do you calculate it? by using the following expression:

MaVa = MbVb, now solving for Vb

Vb = MaVa / Mb

Vb = 25 * 0.1 / 0.2 = 12.5 mL

So, 12.5 mL is the volume required to reach the first equivalence point. To reach the other two, all you have to do is double the amount:

2nd Volume = 12.5 * 2 = 25 mL

3rd volume = 12.5 * 3 = 37.5 mL

Hope this helps.

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