Calculations involving sucrose, molality, molarity, weight, mole fraction





A sucrose solution containing 30gram sucrose (C12H22O11) in 300ml of that solution, the density of the solution is 2.04g/mm. when these are provided: C=12, H=1, O=16, calculate:
(i)                 Molality of sucrose solution 
(ii)               Molarity of  sucrose solution
(iii)             Weight of sucrose solution
(iv)             Mole fraction of sucrose solution 


To answer question (i) and (ii) we should know what is molality and molarity to avoid confusion that may arise. Molality is a measure of the concentration of a solute in a solution in terms of amount of substance in specified amount of mass of the solvent. It is also known as molal concentration. Its common unit in chemistry is mol/kg while molarity which also known as molar concentration is a measure of the concentration of a solute in a solution or any chemical species in terms of amount of substance in a given volume. Its unit in chemistry is mol/L.
Calculations
(i)                 Basis: number of moles per 1000grams or 1 kilogram of the solvent, for us to get weight of the solution, it will be; volume multiply by density(W=V X D) i.e
V=300ml, D=2.04g/mm
W= (300 X 2.04) = 612 grams, now getting weight of the solvent; we subtract the weight of sucrose (30 grams) from the weight of solution (612 grams): (612-30) = 582 grams.
Weight of sucrose in 1000 grams solvent;
(30/582 X 1000) = 51.55 grams, molecular weight of sucrose will be; (12 X 12) + (22 X 1) + (11 X 16) = 342g/mole.
Molality of the sucrose solution is: (51.55/342) = 0.15073 molal.
(ii)               Basis: 1 Litre of the solution, hence this will be;
(30 X 1000)/ (300) = 100 grams,
Molecular weight of sucrose is (as calculated above) = 342 grams/mole,
Therefore molarity becomes; (100/342) = 0.2924 M
(iii)             The weight of sucrose in the solution will be; weight of (sucrose in grams)/(weight of solution in grams) X 100
(30/612) X 100 = 4.902%
(iv)              Mole fraction of sucrose solution will be;
In 300ml, how many moles of sucrose are there? i.e
(30/342) = 0.08772 moles, but in this solution (300ml) there is 582 grams of solvent (water). Therefore, the number of moles of water at (300ml solution) is;
(582/18) = 32.33 moles, a point to note molecular weight of water is 18 grams/ mole.
Hence mole fraction of sucrose is;
(0.09)/ (0.09 + 32.33) = 0.00278

NB: 0.08772 moles equivalent to 0.09 and it is the reason why 0.09 is used in the above calculation and mole fraction = (moles of the solute in the solution)/ {moles of the solvent + moles of solute in the solution}


Comments

Post a Comment

Popular posts from this blog

How to write a Forensic Case Study: Murder of Junko Furuta

Image
Photo of Junko Furuta  For one to score high marks in a Forensic Case Study, one must  adhere to the following: Background of the case: This section contains details about the crime. It includes; the reasons for committing the offense, the conditions which the crime was committed, the circumstances of the crime scene and clear identification of the suspect(s) and the victim(s). One should ensure that the information in this part gives a precise idea of what the case is all about.   Pick and Write an article on Iwriter Website Evidence: This encompasses the piece of evidence found at the crime scene. The evidence should be described in details, that is, ways in which the evidence was collected, processed and preserved.   Forensic Tests: The tests should be two or more that were used to analyze the evidence. The science or theory of instrumentation used must be described fully.   Results: This consists of a detailed explanation of the forensic ...
Read more

Roles of Sodium Carbonate when Extracting Caffeine from Tea Leaves

Image
A student failed to add sodium carbonate (Na 2 CO 3 ) when extracting caffeine from tea leaves with hot water. Though, high yield of caffeine was produced. The crystals melted at temperatures between 202  and 214 degree  Celsius. Explain. This happened because sodium carbonate did not interfere with the caffeine content of the tea leaves. Sodium carbonate (which is a base) deprotonates the phenolic hydroxyl group (-OH) (acidic) of tannins rendering them anionic. Hence, making them highly soluble in water, but parsimoniously soluble in methylene chloride. So, if sodium carbonate is not added when extracting caffeine with hot water, the extracted caffeine would also contain tannins. Additionally, sodium carbonate is added during the extraction to keep caffeine (an alkaloid which is an organic base)  in a free base form. It prevents caffeine from reacting with available acid available, which may lead to the formation cation. It reacts with a phenolic h...
Read more

Arrhenius equation: Increasing temperature increases the rate reaction

Image
The rate of reaction of a particular reaction often increases as one increase the temperature. However, how can this belief be proved using Arrhenius equation? According to the Arrhenius equation, the rate constant (k) is obtained by the ratio of Ea to RT, in which Ea denotes required energy while RT is the available thermal energy. The Arrhenius Equation is given by: k=Ae - (Ea/RT) When Ea is larger, it results in slower reactions. On the other hand, when RT is large, it leads to faster rate of reaction. Larger Ea gives smaller rate constant k, but larger RT produces larger rate constant (k). Larger rate constant increases the rate of reactions.  By using Arrhenius equation, one can easily see that when the temperature is increased, the rate constant will also increase. Let us consider these two scenarios. In the first scenario, Ea=120,000 Joules/mole, R=8.31 mol -1 K -1 , and K=373K.In the second scenario, Ea=120,000Joules/mole, R=8.31 mol -1 K -1 , and K...
Read more