2. In this week\'s lab, we will be using an experiment called a titration to det

Question

2. In this week's lab, we will be using an experiment called a titration to determine how much citric acid is in your favorire (not red or pink) fruit juice. We will perform a titration in which NaOH e) is added to the fruit juice (acid via a buret. A special substance called an indicator will be added to the fruit juice sample before the titration begins. It is called and pink under basic conditions. NaoH solution will be added to the henolphthalein it fruit juice sample until the fruit juice solution turns pink. When the indicator turns pink,we know that the number of moles of NaoH delivered have fully with the citric according to the following equation: sodium hydroxide sodium citrate water citric acid (a) Why would it be a bad idea to use this method of analysis to determine the acid content of traditional cranberry juice? see change our, zuee is nkli red, aud e color of the inolaator is also an (b) What is the name of the molecule that indicates when this titration is complete? How does it indicate? (c) What is the molar ratio between sodium hydroxide and citric acid? What does this mean? 1.3 73

Explanation / Answer

Ans. B. The molecule that indicates the endpoint (completion) of titration is called an indicator. The indicator changes its color upon completion of titration. For example, when mixed with citric acid solution, the indicator phenolphthalein is colorless in acidic condition. When titration is complete with NaOH, the solution becomes slightly alkaline. In alkaline condition, phenolphthalein changes its color to pink- thus indicates the endpoint of titration.

Ans. C Molar ration of NaOH and Citric acid-

            NaOH : Citric acid = 3 : 1

  It means that 3 mol of NaOH is required to neutralize 1 mol of citric acid.

Ans. D. To answer this question, the #1 details should have been provided.

In absence of that information, a hypothetical example is presented here.

Let the molarity of standard NaOH solution = 0.10 M

Molarity of citric acid solution = 0.10 M (you may need to calculate is using given mass of citric acid; or from endpoint value of NAOH, you may be asked to calculate citric acid content/ purity in the sample). For, simplicity, it’s assumed that concentration of citric acid is 0.10 mL.

Titration: The volume of NaOH consumed during titration = 30.0 mL = 0.030L (all calculation are based on this value).

Number of moles of NaOH consumed during titration =

Molarity of NaOH x Volume (in liters) consumed

= 0.10 M x 0.030 L

= (0.10 mol/ L) x 0.030 L

= 0.003 mol

Thus, the titration consumed 0.003 moles of NaOH.

# From “NaOH : Citric acid = 3 : 1”

Note that 3 mol of NaOH neutralize 1 mol citric acid.

So, moles of citric acid neutralized by 0.003 mol NaOH =

                                    (1 /3) x moles of NaOH consumed

                                    = (1/ 3) x 0.003 mol

                                    = 0.001 mol

Thus, 0.003 mol NaOH neutralizes 0.001 mol citric acid.

By doing similar calculations, you can get the result for #1.

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