Question

grams of KHP 0.6413

mL NaOH needed to titrate KhP to endpoint 38.80

Grams of unknown acid: 1.5106

mL NaOH needed to titrate unknown acid to endpoint

33.55

need to know molarity of NaOH solution and Molar mass of unknown acid. Please help and show work!

CHEM 265 LAB EXPERIMENT NUMBER 4 DETERMINATION OF THE MW OF AN UNKNOWN ACID This experiment utilizes a titration to determine the molecular weight of an unknown acid. In this experiment, the endpoint of the titration will not be signaled by an indicator Instead, we will employ potentiometry, which is the use of electrodes to measure voltages in solutions that provide chemical information. To do this, we need an electroactive species whose response we can measure. We also need two electrodes: one which will respond to the electro-active species and another that will develop a constant reference voltage. These two are called the indicator electrode and the reference electrode. Here, we will use the hydrogen ion, as the electroactive species and the pH meter with indicator & reference electrodes as the measurement device. Special combination or glass electrodes have been developed for the pH meter, where both indicator and reference electrodes have been incorporated into a single probe. Glass electrodes have also been developed which respond selectively to other clectroactive species, such as F, C, Na, and others. In performing titrations using indicators, titrant is added until the indicator turns color, indicating the end of the reaction, or to a "dead stop". In a potentiometric titration, you will keep adding titrant until you are well past the equivalence point, determining the equivalent volume of titrant used after-the-fact. A titration curve will be prepared by plotting pH vs. volume of titrant. As the titration begins, you will add titrant, record the volume and measure the pH, add more titrant, again measure pH and so forth until sufficient data has been collected to prepare a titration curve. The endpoint of the titration is determined by graphical means. See pages 202-204 in your text for a more in depth discussion of titration curves. You may ask, "why go to so much trouble when indicators do a fine job in signaling endpoints in titrations?" One answer is that there are chemical reactions that are useful in quantitative analysis but have no applicable indicator to signal the endpoint. Also, even if there is an indicator available, the use of potentiometry allows the automation of the determination, where multiple samples can be run without human intervention A. TITRATION OF ACID STANDARD Here, you will use a primary standard to re-standardize the NaOH solution left over from the previous experiment. Potassium hydrogen phthalate (abbreviated as KHP: formula is KHC,H0 molar mass - 204.221 g) is a solid organic acid which is widely used as a primary standard to determine the molarity of NaOH. DO NOT DRY THE KHP IN THE OVEN. You will be issued an acid unknown which also should not be dried in the oven. NOTE: If the molarity of your NaOH solution (from experiment 3) is outside the range of 0.09 to 0.11 M, ask your instructor for adjusted KHP weights for the next step.

Answer 1

Vol (L) 1st titration in KHP + NaOH -→ Nakp + O. imol inol moles of KIP = mass = 0.64139 Mм = 0.003140 moles moles of NaOH = 0.003140 204.22g/mol moles = molarity x volume w L. Molarity of NaOH = moles = 0.00 3140 mol 38.80ml = 1000 ml 0.6809 mol/L. Titration 2 NaOH x unknown acid. volume of NaOH used = 33.55 mlcomes = 0.03355 L moles of NaOH = 0.0809 mol/l) 0.033554) = 0.002714 moles. Moles of Acid = 0.002714 moles = Mass Molar mass 0.002714 moles = 1.5106 g Molay mass Molaymass = 1.5106 9 0.002714 moles 556.56 alone. Please check the 'mass of acid and 1.51069). Molarity of NaOH = 0.081 M. Molar mass of unknown acid = 556.6 9/mol.