A sample of solution of RbCl (molar mass 121 g/mol) contains 11 percent RbCl by mass. From the following information, what is needed to determine the molarity of RbCl in the solution? The answer key says Mass and volume of sample. I was thinking only volume is needed because you can get the mass from the information given above (11% RbCl by mass in solution). Are you not able to find mass from there?

Question

A sample of solution of RbCl (molar mass 121 g/mol) contains 11 percent RbCl by mass. From the following information, what is needed to determine the molarity of RbCl in the solution?  The answer key says Mass and volume of sample. I was thinking only volume is needed because you can get the mass from the information given above (11% RbCl by mass in solution). Are you not able to find mass from there?

sunnnystrong · Answer

Hmmm.... Okay so recall that
*Molarity = moles of solute/liters of solution

If you have a solution (lets say water & RbCl) & you know the percent by mass of RbCl & the volume of the solution you wouldn't be able to find the mass of RbCl without knowing the density of the solution...

cmkc109 · Answer

@sunnnystrong I recall from somewhere that if it is given a % mass, you can assume total = 100g. 11% mean 11g of RbCl and 99g of H2O

sunnnystrong · Answer

Well... wouldn't you need the mass of the solution?
You would generally use the weight/weight percent of solute to convert directly to the mass of solute... Don't thiink you can assume the mass of solution is always 100g...

cmkc109 · Answer

All right, thanks.