Separation of a Mixture NH4Cl, NaCl, and SiO2

Objectives You will be given a heterogeneous mixture of NaCl, NH4Cl and Sand you must determine its composition The composition can be defined in terms of the mass % of NaCl, NH4Cl and Sand. You are aiming to verify, within 1-2%, the actual mass % of the mixture 𝑚𝑎𝑠𝑠 % 𝑁𝑎𝐶𝑙= 𝑚𝑎𝑠𝑠 𝑁𝑎𝐶𝑙 𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 ×100% 𝑚𝑎𝑠𝑠 % 𝑁𝐻4𝐶𝑙= 𝑚𝑎𝑠𝑠 𝑁𝐻4𝐶𝑙 𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 ×100% 𝑚𝑎𝑠𝑠 % 𝑠𝑎𝑛𝑑= 𝑚𝑎𝑠𝑠 𝑠𝑎𝑛𝑑 𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 ×100%

Separating the different compounds from each other To be able to measure the mass % of each compound we need to be able to separate each component No two chemical compounds have identical physical and chemical properties Can we use their different physical properties to separate them?

NaCl and NH4Cl, Ionic Solids Ionic bonds are much weaker than covalent bonds Ionic Solids are generally soluble in water This occurs because the O in H2O is partially negatively charged and the H partially positively charged The O in water surround the cations as tease them into solution, and the H in water surround the anions and tease them into the water

Solvated Ions The ions are happier in water than they are in NaCl(s) or NH4Cl(s) Both NaCl(s) and NH4Cl(s) are soluble in water

NaCl – Rock Salt Soluble in water Melting point 801 °C Boiling point 1413 °C 2NaCl(l)  Na2(g) + Cl2(g)

NH4Cl – Ammonium Chloride Soluble in water Melting point 338 °C (decomposes) (sublimation) NH4Cl(s)  NH3(g) + HCl(g)

Silica Periodic Covalent Solid (glass) Insoluble in water Melting point 1600-1725 °C Boiling point 2230 °C

Separating NH4Cl from Salt and Sand A bunsen burner reaches around 1100oC in the hottest part of the flame Ammonium chloride sublimes at 338oC NaCl(s) melts at 801oC SiO2 just gets hot! So remove NH4Cl by heating

Separating Salt from Sand Salt being an ionic solid is soluble in water, sand is not We can remove the salt by dissolving the salt in water and decanting off the salt water (extraction) Grams of NaCl w Sand + 10 mL H2O Pour out 9/10 of the salt water grams NaCl removed in extraction Grams NaCl remain with sand 1.0 0.9 0.1g 0.1 0.09 0.01g 0.01 0.009 0.001g 0.001 0.0009 0.0001g

A Scheme for Separating the Components Weigh the mixture M1 If we heat the mixture in the fumehood the NH4Cl will sublimate and leave as NH3(g) and HCl(g) at 338oC Once the NH4Cl(s) has left re-weigh M2 Dissolve the NaCl(s) in the remaining mixture Dry the wet sand and re-weigh M3

Procedure Take an unknown and record the id number of the unknown in your notebook Weigh the evaporating dish and record the mass to 4 decimal places m1 Transfer ALL of the sample into the dish and reweigh m2 In a fume cupboard heat the dish and its contents using a Meker Burner When the sample stops exuding white smoke, stir it with a glass stirring rod – more NH4Cl will probably be emitted. When no more smoke is seen and none deposits on the stirring rod, turn off the burner and let the dish cool Reweigh the dish m3 Calculate the mass % of NH4Cl Check with me before performing extraction Add 10 mL of deionized water to the dish, stir with clean glass stirring rod Being careful to not lose sand, pour out the water into a beaker, allowing water to flow down a glass rod, keeping the rod pushed up against the lip of the dish to prevent sand escaping Repeat this last step 4 times in all. Dry sand with heat lamp, cool and re-weigh m4 Calculate the mass % NaCl