Moles/Stoichiometry
Chemistry Regents June 2010  Question 14 



Written by The Chemistry Wizard

Answer: 3
Why? Molarity is a measure of the concentration of solute present in a solution or in the case of NaCl, the amount of NaCl present in a solution. A molar concentration can be expressed as 1M (one molar) = 1 mol / L (one mole per liter). A mole is the defined as the equivalent number of atoms as would be found in exactly 12g of Carbon 12 (^{12}C). The number of atoms has been found to be approximately equal to 6.22 x 10^{23}. An aqueous solution is one in which water is the solvent. This means any solution classified as aqueous must have used water.
Answering the Questions:
To answer this question, we need to understand the concept of aqueous solution and molarity. However, if these were not known the correct answer could have been determined. The questions ask about an aqueous solution, this should immediately indicate that water is a necessary component of the solution. This means answers (2) and (4) would be eliminated as one of the best possible answers, as they both do not immediately indicate the use of water as the solvent. The question also speaks to molarity; therefore, it would be a safe bet to choose an answer which speaks of molarity rather than grams. This means answer (1) would be eliminated, leaving answer (3) as the best possible answer. 
Chemistry Regents June 2010  Question 28 



Written by The Chemistry Wizard

Answer: (4)
Why? The question asks what process can be used to determine the concentration of a solution by reacting it with a known quantity. This is quantitative analysis and can be solved with knowledge of a few concepts. Firstly understanding the concept of a balanced chemical equation and secondly basic mathematics, to determine the quantities and ratios based on the chemical reaction equation.
A balanced chemical equation, such as the one below, follows the general rule that what ever atoms are on the left hand of the equation, should equal the number of atoms on the right side. This means that the number of atoms, of a particular element, makes up the both the reactant and the product.
HCl + NaOH → NaCl + H_{2}O
The ratio of HCl to NaOH is 1:1. Therefore, every molecule of HCl will require and equal number of molecules of NaOH to react completely. Since the concentration of HCl is known and the volume of HCl used is known in moles, then we can determine the number of moles of NaOH present in any given volume.
The process of elimination could also have answered this question. Even if the term titration is unknown, evaporation, distillation and filtration should be familiar terms. As none of those would have described the question, then titration would have been the only alternative.
Answering the Question:
With the knowledge of quantitative analysis in mind the answers can be analyzed in turn. Answer (1) speaks to the process of evaporation, the phase change that occurs on the surface of liquids, changing them to a gas. However, evaporation should not be confused with boiling which occurs throughout the entire liquid. Needless to say this is not a form of analysis.
Answer (2) is distillation, which is the separation of a mixture of liquids, on the basis of differences in boiling points of the liquids. Again this is a method of analysis, but not a method of determination of concentration through chemical reaction.
Answer (3) is filtration; this is another method of separating two or more substances, however in the case of filtration the separation is between a solid and a liquid. Passing the mixture through a filter apparatus with pore sizes too small for the solid to pass through, but which will allow the liquid to pass does this. As both reactants were in aqueous solution, filtration would not be a suitable method of determination of concentration. This leaves answer (4), titration, which is a method of analysis used to determine the unknown concentration of a known reactant. 
Chemistry Regents June 2010  Question 34 



Written by The Chemistry Wizard

Answer: (2)
Why? Significant figures are used to express the digits of a number that contribute to the precision of the number. As a general rule of significant figures, when multiplying or dividing, the measured number with the least number of significant digits is the one used to determine the significant digits of the calculation.
Answering the Question:
In the above question, two digits have been given: mass equal to 34.261g and volume equal to 3.8cm^{3}. This means the number of significant figures in volume will be the number of significant figures used for the calculation. The number 34.261 has five significant figures, while volume at 3.8 has two significant figures. Volume has 2 significant figures, 3.8, density = ^{mass}/_{volume}. Therefore, the density is 9.01605, however as volume had only two significant figures, when this is applied to density, the value would be 9.0, as this is the calculated value with two significant figures. Therefore, answer (2) is the correct answer. 
Chemistry Regents June 2010  Question 56 



Written by The Chemistry Wizard

Answer: 75g/mol
Answering the Question:
The gram formula mass can be calculated by adding the atomic mass of all the atoms present in the formula.
The atomic mass of:
Carbon: 12
Hydrogen: 1
Nitrogen: 14 and
Oxygen 16
Next the total number of each atom has to be calculated.
2xC = 2x12 = 24
5xH = 5 x1 = 5
1xN = 1x14 = 14
2xO = 2x16 = 32
Finally we determine the sum total of all the atomic masses.
24+5+14+32 = 75 
Chemistry Regents June 2010  Question 65 



Written by The Chemistry Wizard

Answer: The solution would be termed super saturated.
Why? The question says that 80g of KNO_{3 } was dissolved in 100g of water. The chemical equation for this process is shown below:
KNO_{3}(aq) + Heat ↔ K^{+}(aq) + NO_{3}^{}(aq)
The forward reaction is endothermic, meaning it will absorb heat. According to Le Chatelier's principle, if heat is added to the system, there will be a tendency of more dissociated ions to be formed to try and bring the system back to equilibrium. Therefore, the solution would dissolve further.
At room temperature, the equilibrium of the reaction would facilitate far less solute dissolving. The solution at that temperature would be classified as saturated. At higher temperatures the chemical equilibrium shifts, allowing for more solute to dissolve. 
Chemistry Regents June 2010  Question 69 



Written by The Chemistry Wizard

Answer:
Answering the Question:
An appropriate scale for the axis is based on the number of axis lines and the highest and lowest data points on the Yaxis. The aim of the graph is to utilize as much area of the graph area as possible. This allows for the scaling of a graph that is large enough to be properly analyzed. After the scale has been chosen, the given data can be plotted on the graph using the information provided for both the xaxis ("Number of Drops" in this case), and the yaxis ("Time" in seconds). 
Last Updated on Tuesday, 07 June 2011 16:39 
Chemistry Regents June 2010  Question 75 



Written by The Chemistry Wizard

Answer: 2120g of Na_{2}CO_{3}
Why? Mass of sodium hydrogen carbonate = 83
Mass of sodium carbonate = 92
Mass of water = 18
Mass of carbon dioxide = 44
Therefore, for the chemical reaction
2 NaHCO_{3}(s) + Heat → Na_{2}CO_{3} + H_{2}O + CO_{2}
2 x 83 + heat → 104 + 18 + 44
Therefore, 166 → 104 + 18 + 44
And finally, 166 → 166
This indicates that the mass of the reactants is equal to the mass of the product. This can be applied to the given quantities.
If 3360kg NaHCO_{3} were originally used, resulting in 360kg of water and 880kg CO_{2} then the mass of Na2CO3 can be calculated since the reaction went to completion.
3360kg → Na_{2}CO_{3} + 360 + 880
Therefore, Na_{2}CO_{3} = 3360  360 880 = 2120
This means that 2120kg of Na_{2}CO_{3} was produced. 
Last Updated on Wednesday, 08 June 2011 09:38 




