chemestry grd 11

A typical soap molecule consists of a polyatomic anion associated with a cation. The polyatomic anion containd hydrogen, carbon & oxygen. One particular soap molecule has 18 carbon atoms. it contains 70.5% carbon, 11.5% hydrogen, and 10.4% oxygen by mass. It also contains one more alkali metal cation. Determine the identity of the cation. Show your work.

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  1. You know it is C18 something. 18 x atomic mass C of 12 = 18*12 = 216 for the mass of C in the soap.
    70.5% of molar mass = 216; therefore, molar mass must be 216/0.705 = ??
    You know the metal must be
    70.5%C + 11.5%H + 10.4%O + ?%M = 100
    So %M is just 100 - sum of C,H, and O.
    Then %metal x molar mass = atomic mass M

    The problem is confusing to me when it says that ....."It also contains one more alkali cation." I have no idea what that means. It could mean it contains a "second" cation (mixed with the first) or it could mean a different cation (different from what) or something else. Any way, this is the only answer that makes any sense to me. Perhaps I've missed something.
    I think the soap is C18H35O2Na.

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