# Lab Report Voltaic Cell: which masses to use?

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So, I am designing my own procedure on how the concentration of substances affects the voltage in a voltaic cell. I am given two substances: 1) CuO4S.5H2O and the molecular weight is 249.68g/mol.

2) Mg(NO3)2.6H2O and the molecular weight is 256.41g/mol. I have chosen my dependent variable to be the voltage and the independent variable being the concentration of solutions. I have to investigate how the change in the concentration in one of the substances affects the voltage. I am told to find out what mass I should be using in order to come up with different concentrations to use...I am confused about this bit...how do I find out which masses to use?? Oh, and I am going to be using 50mL of each solution.

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There might be some safety parameters [ I haven't worked with those substances], but basically, you can decide which concentrations you want by saying molarity [the concentration] = moles / liter. So if you're going to use 50mL of water [if the adding substance won't add noticeable volume], then Molarity = moles/.050L. & to find moles, you'd say molar mass [molecular weight] for CuO4S.5H2O is 249.68g/mol. So rearranging from before, moles = molarity*(.05L) = mass/molecular weight because mass = molecular weight*number of moles.

So if you want a molarity of .2mol/L and .4mol/L, then you'd say mass=molecular weight*molarity*volume of solvent [and the vol is .05L in this case]. And now that I look over this, there's probably a simpler way to do this. If it's confusing, sorry. I'm running on a low battery & I don't want to recharge.

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Well you need to work out how you're going to make the solutions of different concentrations -- i.e. how much of substance X do you need to stick in to ensure a solution of Y molarity?

You need to work out the molar mass of the crystals you're using and then, allowing for the size of your flask (e.g. 1M in a 1L flask would equate to 0.1M in a 100ml flask), work out how much of it to weigh out and then mix in in order to achieve the concentrations you need. This requires simple use of Mass = Moles x Molar Mass.

So if you wanted 0.1M of whatever it is, you'd say the mass you need to dissolve is equal to 0.1 x Molar Mass.

Just don't forget that the H2O counts as part of the molar mass when you're working that out for your substances.

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