PERCE ENTAGE CO OMPOSITIO ON OF HYDR RATES Reminder – Goggles must m be worn n at all timess in the lab! PRE-LAB B DISCUSSIO ON: Hydrates are ionic com mpounds (salts) that have a definite am mount of waterr as part of their structure. This “water of hydration” is released as vapor v when th he hydrate is heated. The rremaining sollid is known a as the anhydrou us salt. The general g reaction for heating g a hydrate iss: Hydratte
Anhydrous saltt +
water
The Δ sign means that heat is applie ed. The perce ent of water in n a hydrate ca an be found e experimentallyy by accurately y determining g the mass of the hydrate and a the mass of the anhydrous salt. The e difference in n mass is due to the e water lost by y the hydrate. The percentt of water in th he original hyydrate can be calculated ea asily: er x 100 % H2O = Mass of wate Mass of hydrrate In this exp periment, a hy ydrate of copper (II) sulfate e (CuSO4 ⋅ 5 H2O) will be sstudied. The cchange from copper (II) sulfate e pentahydratte to anhydrou us copper (II)) sulfate is acccompanied byy a change in n color. CuSO4 ⋅ 5H2O Blue
CuSO4 + white
5H2O
This inves stigation shou uld aid in the understanding u g of the comp position of hyd drates, simple e decomposittion reactions,, and the Law w of Definite Composition. C PURPOSE: The purpo ose of this lab b is to determine the perce ent of water in n a hydrate. PROCEDURE: 1. Clean n and dry your evaporating dish. 2. Prepa are your ring stand s set-up as a shown in the t diagram a at the right. 3. Place e your empty evaporating e dish d on the wire gauze and d heat the dish h with the hottes st flame for 3 minutes. This s is done to be b sure that th he evaporating dish is absolutely dry. 4. Using g crucible tong gs, remove th he dish from th he apparatuss. Place it on a pressed fiber pad p and allow w it to cool for several minu utes. 5. Using g a balance, find the mass of the cooled evaporating dish. Record d this mass in the Observ vations and Data D section. Never weigh an object wh hen it is hot be ecause heat waves w tend to o be circular and a upward (cconvective), w which tends to make obje ects appear to o weigh less. 6. With the t evaporatin ng dish on the e balance, me easure into it exactly 2.000 0 grams of cop pper(II) sulfate e penta-hydra ate. Be sure the t spatula is very clean to o avoid contam mination. Reccord the mass of the dish and the hydratte in the Data section. e the evaporatting dish and the hydrate on o the wire ga auze. Gently h heat the dish by moving th he burner 7. Place back and forth arou und the base. Increase the e heat gradua ally. Avoid anyy popping and d spattering. 8. Heat strongly for 5 minutes or until all of the blue color hass disappeared. During the e heating, a glass stirring rod may be e used to “spre ead” the solid d and break u up larger portions of the hyydrate. Be surre not to pick up u any of the solid s on the stirring s rod. If the edges of the solid app pear to be turn ning brown, re emove the he eat momentarrily and resum me heating with a cooler fla ame. 9. Allow the evaporatting dish to co ool for two min nutes. Immed diately find the e mass of the e dish plus the e anhyd drous salt, and record the mass m in the data d section. T The dish musst be weighed d before the co ompound reabs sorbs water fro om the atmos sphere. 10. When n you have fin nished weighin ng the dish an nd salt, place e the dish bacck on the coun nter and squirrt some distille ed water onto o the anhydrous salt. Record any chang ge in appearance in the Da ata section. 11. When n you have fin nished, the an nhydrous salt can be throw wn into the wa aste containerrs, and your evapo orating dish washed. w
RESULTS: Data Remember to record masses to three decimal places 1. Mass of evaporating dish _________________g 2. Mass of dish + hydrate
_________________g
3. Mass of dish + anhydrous salt
_________________g
CALCULATIONS Remember to show all of your work. 1. Calculate the mass of water lost from the hydrate. 2. Find the percent of water lost from the original hydrate (remember that percentage is “the part divided by the whole, multiplied by 100”). 3. Calculate the Theoretical yield: This is where we find out how well you did. Theoretical yield is the yield you should have gotten if you had done everything perfect. a. Using the periodic table, add up the weight of the five water molecules b. Next, add up the weight of the entire compound, including the five water molecules. c. Theoretical percent of water (“the part that is water, divided by the weight of the whole compound, multiplied by 100”):
