<p>Hey guys, im currently in h. chem and i have no idea whats going on. We are doing the moles stuff right now and the teacher doesnt explain properly and the book is crap. I need to know how to do these 5 problems for tomm so please help me. You dont HAVE to tell me the answers, i just want to know the steps on how to do them. Please help me, i have a test on these stuff this Thursday and i need help.</p>
<li>45.2g AsPO4 would contain how many atoms Oxygen?</li>
<li>247.9g Yb2(PO4)4 would contain how many grams of Yb?</li>
<li>12.43 mol Er(CO3)3 would contain how many CO3 -2 ions?</li>
<li>How many grams Cr2O7 -2 would be found in 2.45 * 10 to the 25 atoms Hydrogen?</li>
<li>How many grams of C8H18 would you need to measure to get 6.7 mol of Hydrogen?</li>
</ol>
<p>1) Find the molar mass of each element (making sure you multiply the oxygen's by 4). Then divide the 4 oxygens' molar mass by the total to give you percent of oxygen in the compound. Use that percent to find out how many grams of oxygen are in the 45.2 gram sample... then divide this number by the molar mass of oxygen (16), and then to get the actual number of atoms I believe you multiply by avogadro's number (6.02*10^23)</p>
<p>I think I did that right...</p>
<p>2) Follow the first few steps from above. Find the total molar mass of Yb2(PO4)4 and then divide the molar mass of Yb2 by the total to find percent Yb in the compound. Multiply this percent by 247.9.
3) Same as above.
4) Atoms hydrogen? Is that supposed to be part of the problem? Well to find grams of Cr2O7, divide (2.45 * 10^25) by avogadro's number, then multiply this number by the molar mass of Cr2O7.
5) Do the same as problem 2 and 3 to find percent of each element in the compound. Use the percents to find out how many mols of C are needed if you have 6.7 mols of H. Just find the molar mass of each of these in grams and add (6.7 mols H * 1g + x mols C * 12g).</p>
<p>okay the way GoldShadow seemed a bit confusing but i might just be restating what he said</p>
<p>1) for this one you first have to find the molar mass of the compound. You then divide the grams given by the molar mass in order to give you the number of moles of the compound. When you have this number you multiply it by 4 since there are four moles of oxygen in each mole of that compound. Then you simply multiply the number of moles of O by Avogrado's number (6.23 x 10^23).</p>
<p>2) Divide the number of grams given by the molar mass of the compound to find out how many moles it is. Then multiply that number by 2 since there are 2 moles of Yb in each mole of the compound. Then multiply that number by the molar mass of Yb</p>
<p>3) um it seems as if there are 3 moles of CO3 ions in each mole so you multiply the number of moles of the compound by 3 (12.43 x 3). You then multiply this number by Avogadro's number ^.</p>
<p>4) i dont get the part where you say units so i'll leave ya to clarify that</p>
<p>5) hmm im not sure about what the question is asking. Is it the combustion of C8H18 or just like the amount of moles. Also dont forget that H is a diatomic molecule since that can screw you over some times.</p>
<p>In number 4, units means "particles" or "molecules"...</p>
<p>How many grams Cr2O7 -2 would be found in 2.45 * 10^27 units of Al2(Cr2O7)3?</p>
<p>So do the same thing here and find the percent of Cr2O7 in the compound (keeping in mind there are 3 molecules of it). Now keep this percent in mind. Divide (2.45 * 10^27) by avogadro's number to get the number of moles of Al2(Cr2O7)3, then multiply by the molar mass of Al2(Cr2O7)3 (which you should have done when finding percents). Now you have total grams of Al2(Cr2O7)3... and to find grams of (Cr2O7)3 multiply the percent (Cr2O7)3 you found earlier by the mass of Al2(Cr2O7)3.</p>