Question #ba790

I'm going to assume that you must determine the number of formula units presents in #"1.25 mL"# of a sodium hydroxide solution of known molarity #c#.

The idea here would be that you can use a solution's molarity as a conversion factor that allows you to figure out the number of moles of solute present in a given volume of solution.

Molarity is defined as the number of moles of solute, which in your case would be sodium hydroxide, present in #"1 L"# of solution.

So, let's say that your sodium hydroxide solution has a molarity of #c# #"mol L"^(-1)#. This means that every #"1 L"# of solution contains #c# moles of sodium hydroxide.

You can thus use this ratio as a conversion factor to determine the number of moles of sodium hydroxide present in

#1.25 color(red)(cancel(color(black)("mL solution"))) * (1color(red)(cancel(color(black)("L"))))/(10^3color(red)(cancel(color(black)("mL")))) * (c color(white)(.)"moles NaOH")/(1color(red)(cancel(color(black)("L solution")))#

# = (0.00125 * c)color(white)(.)"moles NaOH"#

To convert this to formula units of sodium hydroxide, use Avogadro's constant

#(0.00125 * c) color(red)(cancel(color(black)("moles NaOH"))) * overbrace((6.022 * 10^(23)color(white)(.)"formula units NaOH")/(1color(red)(cancel(color(black)("mole NaOH")))))^(color(blue)("Avogadro's constant"))#

# = (7.53 * c) * 10^(20)color(white)(.)"formula units NaOH"#

Keep in mind that this is just my interpretation of what the question would actually ask.