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8. Consider a sample of calcium carbonate in the form of a cube measuring 1.25 in on each edge. If the sample has a density of 2.71 g/cm 3, how many oxygen atoms does it contain? 1 in = 2.54 cm see attached - does it look ok? |
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| | From:  ·Steve· | Sent: 10/13/2005 6:59 AM |
Looks OK except that you do not need to calculate the weight percent of oxygen. Starting from the point where you have 86.74 g of CaCO3 (the weight of the cube), do the following: 1) convert to moles of CaCO3 by dividing by 100.0869 g/mol
2) moles of O = moles of CaCO3 X 3 mol O / mol CaCO3
3) multiply moles of O by 6.022 X 1023 atoms of O / mol O
The percentage route gives the same answer, just adds an extra step or two! 
Steve
>> 8. Consider a sample of calcium carbonate in the form of a cube measuring 1.25 in on each edge. If the sample has a density of 2.71 g/cm3, how many oxygen atoms does it contain? 1 in = 2.54 cm. See attached - does it look ok? << |
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Thanks a lot, Steve! I see you correct my sig figs sometimes. Should I use extra SF's while doing my problems and then narrow it down at the end? It seems sometimes my answer is off a little. What about when you have addition or subtraction in the middle of a multiplication/division? Thanks |
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One more question: why is N 2O 4 nitrogen tetroxide and not dinitrogen tetroxide? |
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| | | From: ·Steve· | Sent: 10/13/2005 9:24 PM |
>> Should I use extra SF's while doing my problems and then narrow it down at the end? << That's the best way. I try to just let the numbers ride on my calculator, or put them in memory if I need to recall them later. As long as you don't round too much, roundoff error will be low. I generally follow the same principle for all operations, including addition and subtraction. But you can lose significant figures in a hurry when a subtraction is present, for example 6.789 g = 6.789 g = 7.543333333 g/mL or 8 g/mL rounded to one significant figure. 10.9 mL - 10.0 mL 0.9 mL That is, 8 ± 1 g/mL, a range that still includes the unrounded answer. Nevertheless, one sig. fig. answers always leave me with a feeling of unease! Steve |
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| | | From: ·Steve· | Sent: 10/13/2005 9:54 PM |
Dinitrogen tetroxide is the "proper" name for N2O4. Frequently, the rules are allowed to "slide" a little. For example, P2O5 is properly called diphosphorus pentoxide, but it is usually just called phosphorus pentoxide because it is a common, well-known chemical. H2S should properly be called dihydrogen sulfide, but since this is the only common compound composed of these elements, it is usually just called hydrogen sulfide and everyone knows what it is. There is no other common oxide of nitrogen that has 4 oxygens, so anyone with some familiarity with the subject would understand when you say "nitrogen tetroxide" that you mean N2O4 rather than "NO4". But when there are other, common possibilities, the name has to be specific. If we say "carbon oxide" or "sulfur oxide", it would be unclear whether we were talking about CO or CO2, or SO2 or SO3.  Steve |
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We are naming compounds. My teacher wanted us to write formula for nitrogen tetroxide. I thought an N around 4 O's did not look right, so I looked it up and did not find this. And according to what you say there is no NO4 - but that is what she thought it should be. So I guess she was wrong. It is confusing because it was from an actual test she gave to her class last year (our practice test this year). Also you said, """H2S should properly be called dihydrogen sulfide.""" That would be the gas or other non aqueous form. In water this would be hydrosulfuric acid right? Also how do you know when to treat H as a metal or non metal when naming compounds? This can make a difference in naming the compound as an ion or a molecule. Like H2O2 it is hydrogen peroxide so H must be a metal otherwise it would be dihydrogen dioxide. And then you have MgH2 where H acts as an anion. Are there any other things to watch out for when naming? One more thing - do semi-metals follow any pattern? Do most semi-metals and non-metals, when combined, make molecular compounds? Thanks JB |
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| | | From: ·Steve· | Sent: 10/15/2005 9:21 AM |
The known oxides of nitrogen are NO, NO2, NO3 (unstable), N2O, N2O3, N2O4, N2O5, and N2O6 (also unstable). Strictly speaking, nitrogen tetroxide would be NO4 (even though it doesn't actually exist as far as I know), so your teacher is right about that. It is perfectly correct to say, "Nitrogen tetroxide (meaning NO4) is an unstable molecule not known to exist." Unfortunately these names are often used carelessly, with people calling N2O4 "nitrogen tetroxide" when they should really call it dinitrogen tetroxide. A Google keyword search on "nitrogen tetroxide" gives hits mostly for N2O4. (Sometimes NO4 is mentioned specifically, but I'm not so sure about that!) There is a Wikipedia encyclopedia entry for "nitrogen tetroxide". It says (with some typos), Nitrogen tetroxide From Wikipedia, the free encyclopedia. Nitrogen tetroxide (N2O4) is a hypergolic propellant often used in combination with a hydrazine-based rocket fuel. Nitrogen tetroxide is an oxidizer and highly toxic and corrosive. When it is combined with water the resultant is nitric acid. It is also called dinitrogen tetroxide. And, Talk:Nitrogen tetroxide From Wikipedia, the free encyclopedia. According to the rules for naming chemical elements, the name that this article designates the element at indicates NO4 (one nitrogen and 4 oxygen.)Does NO4 exist?? It can't! The reason is: The oxidation number for oxygen is -2. To cancel out the 4 oxygen atoms, the nitrogen has to be 4 * 2 = 8. Nitrogen can have an oxidation number of +5, and so 8 is too large. The following compounds exist: - N2O: Dinitrogen monoxide
- NO: Nitrogen monoxide
- N2O3: Dinitrogen trioxide
- NO2: Nitrogen dioxide
- N2O5: Dinitrogen pentoxide
>> In water this would be hydrosulfuric acid right? <<
That's normally the case. In aqueous solution, H2S is a weak acid, so it is usually appropriate to use the acid name in this context.
>> Also how do you know when to treat H as a metal or non metal when naming compounds? <<
With the most active metals (Group IA and IIA) hydrogen can form ionic hydrides in which it is more energetically favorable for H to be the negative ion. In any ionic compound the positive ion is always named first and the negative ion second. In NaH and similar hydrides, the hydrogen is the negative ion. In binary molecular compounds like HCl, the rule is to name the less electronegative element, which has a "partial" positive charge (d+), first, and the more electronegative element, which has a partial negative charge (d-), is named second analogous to the way ionic compounds are named. When dissolved in water, HCl actually is in the form of H+ and Cl- ions. In the case of NaH, a table of electronegativities also tells us that H is the more electronegative element.
Names like "water", "hydrogen peroxide", and "ammonia" are common names of these substances and as such are normally used in place of "dihydrogen monoxide", dihydrogen dioxide", and "trihydrogen nitride". One more note - Organic hydrocarbons such as CH4 and C2H6 (methane and ethane) have their own special names and naming rules, different from that described above. >> Do most semi-metals and non-metals, when combined, make molecular compounds? <<
That's correct. Examples would be SiCl4 and GeCl4 which are molecular.
Hope all this isn't too confusing!
Steve
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