Brother Gregory Investigates End
End Osmosis
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Research Assistant Wanted


Brother Gregory wants you to help him find answers to various questions about the physical structure of matter, atoms, and related subjects. In these investigations he will ask you a question and give you the tools to find the answer. You then become his research assistant. You must carry out the experiment, gather data, analyze your results and give Brother Gregory the answer he seeks.

Hint: if you get stuck - ask Mendel's Mother!


Sausages and Molecular Weights

Sausages and Molecular Weights

B "Brother Victor, I need some of your sausage skins," Brother Matthew announced bursting into the monastery kitchen.
"What would you like in them?" replied the hospitalier, without looking up from his pastry, "Beef, pork or your favorite parek v rohliku?"

"Nothing," Brother Matthew told him, "it is one of Mendel's ideas. He wants some pieces of plain sausage skin for one of his science experiments."
"What is he going to do now, find out the color of the pigs eyes from the texture of its intestines?" Brother Victor laughed, sending shower of flour around the kitchen. Brother Gregory's exploits in genetics were famous among the monks of his order.

"Almost that bad," Brother Matthew couldn't help laughing along with his friend, "He thinks he knows of a way of determining the molecular weight of an unknown substance by allowing water molecules to move through sausage skin!"
Both monks collapsed with mirth at the suggestion.

"How is he going to do that?" coughed Brother Victor between bursts of laughter. Sometimes his friend Brother Gregory had the strangest ideas.
"He's read about a hypothesis put forward by a physics professor in Turin about 50 years ago (1811 actually), that equal volumes of all gasses at the same temperature and pressure contain identical numbers of particles. He has somehow combined that idea with a more recent one by Willam Prout that all molecular weights are multiples of that of hydrogen, to come up with the idea that he can determine the molecular weight of an unknown substance by watching water move through sausage skins." Once again the two monks could not help themselves and collapsed laughing around the kitchen table.

Molecular Weight
The molecular weight of a molecule, such as the carbon dioxide seen here, is calculated by adding together all the individual atomic weights.

"Do you think he can do it?" Brother Victor laughed.
"With Mendel you can never be sure," Brother Matthew said, slowly as he began to think about the problem from a scientific point of view. "He has had a lot of funny ideas since he came to the monastery, and a lot of the time he has been proved right. I wouldn't bet against him."

"But what is he going to do?"
"Let's take him the sausage skins and find out," said Brother Matthew, and so they did.


Tools of the Trade

"I got the idea when I was watching Brother Victor clean out the pigs intestines to use as sausage skins," Brother Gregory told his two friends when they asked him. "I saw him put sweet water inside the sausage skin and then drop them in plain water. If he left them there for some time they would expand and expand, apparently by taking in more water through the membrane."

sausage in a bowl

"Yes, I've notice that," said Brother Victor, handing over more of the sausage skins, "whenever I put sugar water inside the sausage skin, and then put them in plain water, they always blow up like balloons. What do you think is happening?"

"Osmosis!" said Brother Gregory triumphantly.
"Eh?" said Brother Victor, but Brother Matthew was already looking thoughtful.
"You mean that the sausage skin is acting like a semi-permeable membrane?" he said, "it is preventing the sugar molecules from passing through it, but allowing the water molecules to move freely?"

"Exactly," exclaimed Brother Gregory, "when ever you get a barrier separating two solutions of water and other substances, and the water can move through the barrier, but the other molecules cannot, then you get this phenomenon called osmosis"

B "You'll have to explain that one," said Brother Victor. He was no scientist and often got left behind when his two friends suddenly got inspired and began taking large leaps in the imaginations. This, he realized, was how a scientific investigation often got started, but he needed help in understanding the basics.

Mendel looked around the kitchen and then picked up a wineglass and Brother Victor's flour sifter or sieve. He poured flour into both and the three monks watched the results.
"Look," he said, "the wineglass holds the flour because the glass is an impermeable barrier - nothing can get through it. But the flour is being sifted through the sieve ..."

"And all over my floor!" shouted Brother Victor, but his friends shusshed him into silence as Mendel continued.

Semi Permeable

"... because the wire mesh of the sieve is permeable to the small flour grains and they can pass easily through the tiny holes." He added some small chunks of bread to the contents of the sieve, and again they watched as the flour trickled through the tiny holes in the wire mesh, but this time the larger chunks of bread were trapped inside the sieve and did not end up on Brother Victor's floor. "The sieve is acting as a semi-permeable barrier allowing the flour grains to pass through it, but not allowing the larger bread chunks to pass through it," Mendel said. "At a molecular level, the sausage skins are behaving the same way. They are allowing the small water molecules to move through them, but are not allowing the larger sugar molecules to pass through them!"

He made it sound as if this explained everything, but Brother Victor was still puzzled. "I see what happens with the flour sifter," he said, "but why are the water molecules moving into the sausage skins when I soak them in plain water. Why doesn't it sift out like the flour?"


A Matter of Movement

"Have you ever heard of a Scottish scientist called Brown?" asked Mendel, but his friends just shook their heads. "I'm not surprised," he went on, "just last year (1863) his observations on the jiggling of pollen grains in water was explained as the result of the rapid movement of water molecules, randomly and at very high speeds."

"Water is moving?" asked Brother Victor, picking up a beaker and looking at the fluid closely.
"At the molecular level, all the water molecules are moving very fast and in all directions," Mendel said with confidence, this was the latest finding concerning the nature of matter, and the difference between solids, liquids and gasses.

Browniam motion

"Inside the sausage skins there are rapidly moving water molecules that jiggle and move all over the place, and equally, outside the sausage skins there is even more water jiggling, moving and spreading itself everywhere. When one of these water molecules bumps into the sausage skin it can pass through it to the other side, without being stopped by the barrier. But ..."

Water and Holes

"When a sugar molecule hits the sausage skin barrier, it cannot pass through!" shouted Brother Matthew, suddenly grasping the concept. "It remains trapped on one side of the barrier. That is why the barrier is called semi-permeable!"

"I think I understand that," said Brother Victor, doubtfully, "but I still don't see how or why water moves inside the sausage skins when there is plain water on the outside and there is a solution of sugar dissolved in water on the inside." "It all depends on the number of times the water hits the barrier," said Mendel, "or the concentration of water on each side of the semi-permeable barrier."
"Let's say you put pure water on one side of the barrier. It's 'concentration' is 100%. Every molecule that hits the barrier from that side is a water molecule, and let's say that every water molecule that hits the barrier passes through to the other side. This is probably not true, but it makes it easier to understand. "

"Not for me," groaned Brother Victor, who was beginning to regret asking for this explanation, "but go on."

"If 100 water molecules are hitting the barrier at any one moment, then all 100 of them pass through to the inside." Brother Gregory paused, and Brother Victor nodded, so he went on.

"But on the other side of the barrier, the inside of the sausage skins, you have two kinds of molecules: water and sugar. Let us say that there are 20 molecules of sugar for every 80 molecules of water, the water is no longer at a concentration of 100%."

"It is at a lower concentration," Brother Matthew added.

Concentration effects
Note: Brother Matthew is only considering the 'concentration' of the water molecules. Most scientists usually talk about the concentration of the solute in the solvent (water in this case). Which means as a solution becomes more concentrated in solute it becomes less concentrated in solvent (water).

"So, if there are 100 molecules hitting the barrier from the inside, at any one time, only 80 of them are water molecules. Let us say that all of them go through the barrier ..."

"There are still only 80 water molecules moving from the inside to the outside," said Brother Matthew.

"While, at the same time, there are 100 molecules moving from the outside to the inside. So, if you do the subtraction -

Moving from the outside to the inside = 100 water molecules.

Moving from the inside to the outside = 80 water molecules.

Net movement of water molecules is 100 - 80 = 20 water molecules.

So, 20 more molecules of water move from the outside to the inside under these circumstances.

"This happens every second, every minute that the sausage skins with their sugar water stay in the pure water, so the contents of the sausage skins expand and expand," said Brother Gregory.

"And this is called osmosis?" asked Brother Victor.

"Yes," his two friends said together.

"Then how does this help you find out the molecular weight of the sugar?"

"I'll have to get my research assistants working on that one," said Mendel, shaking his head. "I think it has to do with this discovery of Professor Avogadro. He worked out the relationship between the molecular weight of an unknown substance and number of molecules in a liter of water."

For an answer the question "Avogadro and Moles?", and what you are going to do, you'll have to ask Mendel's Mother.

Ask Mendel's Mother How do you find
Molecular Weights by
Watching Osmosis?

... it all depends on Avogadro!

© 1999 Professor John Blamire
Science at a Distance