100%(1)1 out of 1 people found this document helpful
This preview shows page 1 - 4 out of 10 pages.
_______________________________EXPERIMENT 4Preparation of Compounds andDetermination of Formulas_______________________________
EXPERIMENT #4PREPARATION of COMPOUNDS and DETERMINATION of FORMULASOBJECTIVESCompounds are chemically combined elements which can be prepared in avariety of ways, such as direct combination of the elements, reactions of mono- andpolyatomic ions, and even through decomposition of other compounds.We hope toprepare two compounds and then to determine the simplest chemical formulas for thecompounds.BACKGROUNDBinarycompounds contain two elements, although they can be sometimes becombined in a variety of ways.For example, if carbon is burned in air (which containsoxygen), there are two common reactions:2 C(s)+O2(g)⇒2 CO(g)carbon monoxide (limited amount ofoxygen available)C(s)+O2(g)⇒CO2(g)carbon dioxide (excess amount ofoxygen available)Some of these reactions occur spontaneously.If sodium (Na) metal is placed ina container of chlorine gas (Cl2), a vigorous reaction occurs producing a white powderwhich we know as salt or sodium chloride (NaCl).Other reactions occur with someprodding, such as increasing the temperature or providing a source of energy like aflame; this is true for the reactions of carbon and oxygen above.Binary compounds areusually combinations of a metal and a non-metal or of two non-metals.Two metals donot usually produce a binary compound.Ternary(three elements)compounds [or those with more than three elements]usually are not prepared by direct combination of the elements.They might beprepared by reactions of positive and negative ions.An example might be silverchromate.2 Ag+(aq)+CrO42-(aq)⇒Ag2CrO4(s)This would not form spontaneously if we place silver and chromium metals in contactwith oxygen gas.Once prepared or discovered, chemists want to know the formula of thecompound.At first, this was done in terms of weight ratios or percentages of theelements, but now formulas are expressed in terms of moles of each element, whereonemoleof an element is equivalent to its atomic mass in grams.In this experimentwe will determine the formula of two compounds and our result will be itsempiricalformula.
Theempirical formulais defined as the lowest whole number ratio of moles ofelements in a compound.This is sometimes the same as the actual formula of themolecule, but sometimes it is not.For example, hydrogen (H) and oxygen (O) form twocommon compounds:water (H2O) and hydrogen peroxide (H2O2).These are themolecular formulas.⇒One moleculeof water contains two atomsof H combined with one atomof O.⇒One moleof water contains two molesof H combined with one moleof O.⇒One moleculeof hydrogen peroxide contains two atomsof H and two atomsof O.⇒One moleof hydrogen peroxide contains two molesof H and two molesof O.If we examine the lowest whole number ratio of moles of H to O (there’s a pun if readaloud!), for water the ratio is 2 H:1 O or 2 : 1 or H2O so the empirical and molecularformulas are the same.For hydrogen peroxide, the ratio is 2 H:2 O or 1 : 1 or HO, so