Lemon juice is acidic because of the presence of citric acid, a weak triprotic a

Question

Lemon juice is acidic because of the presence of citric acid, a weak triprotic acid with the formula (molar mass 191.12 g) and structure shown above. The acidity of a lemon varies with both the variety and the growing conditions with the average being approximately 5% citric acid. This percentage is represented as grams of citric acid per 100 mL of lemon juice. Available equipment and supplies (not all will necessarily be used) Re-draw the citric acid structure on your own paper and circle the hydrogen atoms that are able to ionize. Write a complete balanced molecular equation to show the neutralization of the citric acid by sodium hydroxide. When the NaOH solution was standardized, 1.2582 g of potassium hydrogen phthalate (KHP), a monoprotic weak acid (molar mass = 204.22 g) neutralized a 22.43 mL sample of the sodium hydroxide solution. Determine the molarity of the sodium hydroxide solution. Design an experiment to determine the citric acid content of a sample of lemon juice. List the steps of the lab procedure. Describe all calculations that would be necessary in this experiment. Many commercial lemon-flavored drink mixes include both sodium citrate and citric acid. Suggest a reason that the sodium citrate is included.

Explanation / Answer

C) Density of KHP at room temperature = 1.64 gm/mL
volume of KHP = 1.2582 gm / 1.64 gm/mL = 0.76719 mL
   mol of KHP = 1.2582 gm / 204.22 gm/mol = 6.1610*10-3 mol
volume of NaOH = 22.43 mL
mNaOH VNaOH  = mKHP VKHP
  mNaOH * (22.43 / 1000) L = (6.1610*10-3 / 7.6719*10-4) mol/L * (7.6719*10-4)L
   mNaOH * 0.02243 L = 6.1610*10-3 mol
   mNaOH = 0.2746 mol/L = 0.2746M

e) molarity and volume of base used must be known.
   volume of citric acid used in the experiment must be measured
    mbase Vbase = mCA VCA , use this formula to calculate molarity of citric acid

f) Sodium citrate is used to add limy, citrus flavor to cold drinks. It is also added as preservative and as an
   agent which maintains acidity of the soft drinks.

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