How many cations are there in 10.0 g of sodium phosphate?

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a. 3.67 x 10^22 cations

b. 1.10 x 10^23 cations

c. 9.87 x 10^24 cations

d. 2.96 x 10^25 cations

i think its a but then again i ꜱᴜcκ at chemistry…lol.. can anyone help?

1 Answer

  • First: Sodium Phosphate: Na3PO4

    Na- 22.9 g/mol

    P- 30.9 g/mol

    O- 16.0 g/mol

    Molecular Weight Na3PO4: 163.6 g/mol

    Number of molecules per mole of anything: 6.02 x 10^23 molecules

    Number of Cations per molecule of Na3PO4: 3

    Calculation: (the units of each value are in [square brackets] )

    10[g] / 163.6 [g/mol] 6.02 x 10^23 [molecules/mol] 3 [cations/molecule] = 1.10 x10^23 [cations]

    when you take away the numbers from the problem, it becomes both more general, and more understandable.

    [g] / [g/mol] [molecules/mol] [cations/molecule] = [cations]

    and to put it only in terms of multiplication:

    [g] [mol/g] [molecules/mol] * [cations/molecule] = [cations]

    Break it down into more parts/ three smaller problems:

    [g] * [mol/g] = [mol]

    [mol] * [molecules/mol] = [molecules]

    [molecules] * [cations/molecule] = [cations]

    The important part about stoichiometry is to make sure your units all cancel.

    The answer is therefore (B) 1.10 x 10^23

    PS: I hope this isn’t just a homework question. If it is, I recommend finding more problem sets on stoichiometry online. I put one that looked good in the source list, but I recommend searching stoichiometry problems/problem sets and finding more.

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