![]() ![]() In the presence of Ag + ions that act as a catalyst, the reaction is complete in less than a minute. In the process, the chromium atoms in some of the Cr 2O 7 2− ions are reduced from Cr 6+ to Cr 3+. When a measured volume (52.5 mL) of a suspect’s breath is bubbled through a solution of excess potassium dichromate in dilute sulfuric acid, the ethanol is rapidly absorbed and oxidized to acetic acid by the dichromate ions. The reaction used in the Breathalyzer is the oxidation of ethanol by the dichromate ion: 3CH 3 CH 2 OH(aq) + 2Cr 2 O 7 2 – yellow-orange (aq) + 16H + (aq) → H 2 SO 4 (aq) Ag + 3CH 3 CO 2 H(aq) + 4Cr 3+ (aq) + 11H 2 O(l) green The Breathalyzer is a portable device that measures the ethanol concentration in a person’s breath, which is directly proportional to the blood alcohol level. Higher levels cause acute intoxication (0.20%), unconsciousness (about 0.30%), and even death (about 0.50%). In almost all US states, a blood alcohol level of 0.08% by volume is considered legally drunk. What mass of solid lanthanum(III) oxalate nonahydrate can be obtained from 650 mL of a 0.0170 M aqueous solution of LaCl 3 by adding a stoichiometric amount of sodium oxalate?īecause the consumption of alcoholic beverages adversely affects the performance of tasks that require skill and judgment, in most countries it is illegal to drive while under the influence of alcohol. 0 g AuĪt a 2011 market price of over $1400 per troy ounce (31.10 g), this amount of gold is worth $1170. The problem asks for the mass of gold that can be obtained, so we need to convert the number of moles of gold to the corresponding mass using the molar mass of gold: mass of Au = (moles Au)(molar mass Au) = 0. 132 mol –ī Because the coefficients of gold and the − ion are the same in the balanced chemical equation, if we assume that Zn(s) is present in excess, the number of moles of gold produced is the same as the number of moles of − we started with (i.e., 0.132 mol of Au). We can adapt Figure 4.10 "An Expanded Flowchart for Stoichiometric Calculations" for this particular problem as follows:Īs indicated in the strategy, we start by calculating the number of moles of − present in the solution from the volume and concentration of the − solution: moles – = V L M mol/L = 400. ![]() To calculate the mass of gold recovered, multiply the number of moles of gold by its molar mass.Ī The equation is balanced as written, so we can proceed to the stoichiometric calculation. Then calculate the number of moles of − present by multiplying the volume of the solution by its concentration.ī From the balanced chemical equation, use a mole ratio to calculate the number of moles of gold that can be obtained from the reaction. Given: chemical equation and molarity and volume of reactantĪ Check the chemical equation to make sure it is balanced as written balance if necessary. What mass of gold would you expect to recover from 400.0 L of a 3.30 × 10 −4 M solution of −? Gold is then recovered by reduction with metallic zinc according to the following equation: Zn(s) + 2 −(aq) → 2−(aq) + 2Au(s) Gold is extracted from its ores by treatment with an aqueous cyanide solution, which causes a reaction that forms the soluble − ion.
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