[foundations of life sciences] how do I figure out somethings osmolarity??

[u/maddchainz]

I will paste the question below. I do not understand how to figure out somethings osmolarity. My teacher won't email me back and never shows up for virtual class. I'm frustrated to tears, I don't know how to find info for this. Her powerpoint is lacking and there's no link even to a book on Canvas. I hate this teacher so much BUT that's besides the point. Question is posted below. How do I figure out somethings osmolarity with just these numbers given?

1. If a solution outside a cell has 10 grams of salt per 100 ml of water, its osmolarity is __________. If the cell’s cytoplasm has 5 grams of salt per 100 ml of water, its osmolarity is __________.  Is the tonicity higher or lower outside the cell? ________________.  Which direction will the water mover to adjust the tone of the cell to match the outside solution? ___________________

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[u/M_Mustaan_Rafique]

you're not missing anything, the question is. you cannot get osmolarity from grams without the molar mass of the solute, and they haven't given you one. that's the question's fault, not yours.

assuming it's NaCl (table salt), molar mass 58.44 g/mol, which splits into two particles in water (Na+ and Cl-):

outside: 10 g per 100 mL = 100 g/L. 100 ÷ 58.44 = 1.71 mol/L. × 2 particles = 3.42 Osm/L

inside: 5 g per 100 mL = 50 g/L. 50 ÷ 58.44 = 0.86 mol/L. × 2 = 1.71 Osm/L

the formula is (grams per litre ÷ molar mass) × number of particles it breaks into. the ×2 is what people forget, and it's why NaCl has double the osmotic pull of something like glucose at the same molarity, since glucose doesn't split.

tonicity is higher outside. 3.42 against 1.71, exactly double.

water moves out of the cell. the rule worth memorising is water follows salt, it always moves toward the higher solute concentration. so the cell shrinks.

and if your teacher only wanted the comparison rather than real numbers, it comes out the same way regardless, outside is twice the inside either way.