If the vapor pressure above a 50% mixture of methanol with butanol at 22° C were collected and placed into a flask where it condenses, what is the mole percent of methanol in vapor above the new (second) flask? [Pure methanol has a vapor pressure of 87 torr and pure butanol has a vapor pressure of 29 torr at 22° C]
A. Less than 25%
B. Between 25% and 50%
C. Between 50% and 75%
D. Greater than 75%
D) Greater than 75%
From a 50% by moles mixture of the two liquids, the vapor due to methanol above the first flask is 87/(87 + 29). When the vapor is condensed, collected, and placed into the second flask, the condensed solution is 75% methanol by moles. The vapor pressure of methanol above the second flask is greater than 75%, because methanol evaporates more readily than butanol evaporates. This question presents the principle behind fractional distillation. By continuously condensing and re-distilling, the distillate becomes richer with regard to the component with the lower boiling poing.
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I guess I don't understand the math here or why we use this math equation for this type of problem, (forgive me, my math skills are poor because my last math course was over 8 years ago). When I see this problem, why am I supposed to intuitively think that I need to do 87 / (87 + 29)? Maybe this is because I have not reviewed the fractional distillation section of my review books yet, and this problem came up in a phase change section.
87 torr / (87 torr methanol + 29 ethanol) = 87 / 116
(116/116) = 100% > (87 / 116) > 58/108 = 50%
87/116 = 0.75 = 75%
A. Less than 25%
B. Between 25% and 50%
C. Between 50% and 75%
D. Greater than 75%
D) Greater than 75%
From a 50% by moles mixture of the two liquids, the vapor due to methanol above the first flask is 87/(87 + 29). When the vapor is condensed, collected, and placed into the second flask, the condensed solution is 75% methanol by moles. The vapor pressure of methanol above the second flask is greater than 75%, because methanol evaporates more readily than butanol evaporates. This question presents the principle behind fractional distillation. By continuously condensing and re-distilling, the distillate becomes richer with regard to the component with the lower boiling poing.
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I guess I don't understand the math here or why we use this math equation for this type of problem, (forgive me, my math skills are poor because my last math course was over 8 years ago). When I see this problem, why am I supposed to intuitively think that I need to do 87 / (87 + 29)? Maybe this is because I have not reviewed the fractional distillation section of my review books yet, and this problem came up in a phase change section.
87 torr / (87 torr methanol + 29 ethanol) = 87 / 116
(116/116) = 100% > (87 / 116) > 58/108 = 50%
87/116 = 0.75 = 75%