How to calculate molality with density and molarity?

Chemistry molarity and molality?

• quick question... calculate the molarity and the molality of an NH3 solution made up of 30.0g of NH3 in 70.0g of water. The density of the solution is 0.982g/ml it would be great if you could post the steps on how you did it as well. please and thank you~

• Answer:

  The molarity is 0.02 mol/L. The equation for molarity is moles of solute/ volume of solvent in liters. So you need to know here that 30.0 g is the mass of the solute and 70.0 g is the mass of the solvent. What you need to do is convert the 30.0 g into moles. So it’s like these 30.0 g ×1mol= 1.76 mol(because the grams cancels out) 17 g So how did I get 17 g, it is the molar mass of NH3 which is N(mass of 14) + H3(mass of 1)=14+1×3=17 g. Next you convert 70.0 g(the solvent) into L. Look how I did these: 70.0 g × 1 mole × 22.4 L = 87.11 L (in this case grams and moles cancels out) 18 g 1 mole How did I get 18 grams ? the same as the case from above it is the molar mass of H2O so it is like H (molar mass of 1) + O (molar mass of 16) which is 1 ×2(2 because there are 2 hydrogens) + 16 = 18 g. 22.4 liters is like the universal equivalent of 1 mole so that is why I used it. And then because you now have the right conversions divide 1.76 mol by 17 g and that equals to 0.02 mol/L 2. The molality is 25.14 mol/kg So, the equation for molality is moles of solute/ kg of solvent. We already got the moles of solute in the equation above so it is 1.76 mol. So we just need to convert 70.0 g H2O( the solvent) into kg. Just convert 70 g into kg and that gives us 0.07 kg. then divide 1.76 mol by 0.07 and that is 25.14 mol/kg