<p>Let me walk you through</p>
<p>Convert all of these hydroxide compounds to moles. For example, for NaOH, do 4.00g X (1 mol/40 g)= 0.1 moles NaOH</p>
<p>Do the same for the other two hydroxide compounds and get 0.1 moles KOH and 0.01 moles RbOH.</p>
<p>Since there is only one hydroxide molecule per each compound and they are all strong bases (i.e. dissociate completely), you don't have to worry about doing more stoichiometry. For example, 2 moles of Ba(OH)2 would result in 4 moles of OH-.</p>
<p>Therefore, there is a 1:1 stoichoimetric relationship between moles of compound and moles of OH-.</p>
<p>So now add them up to get 0.21 moles OH-. Now, molality is moles of solute/Kg of solvent. We have 90.0 grams of water and therefore 0.09 Kg of water.</p>
<p>Now do 0.21 moles/0.09 Kg to get molality. The answer is A. 2.33 m. You should be able to do this without a calculator on the AP chem exam FYI.</p>