Please help!!! I don’t understand how to do these! A walk thru would be perfect for both, thanks!!!

18.) A chemical reaction produces formaldehyde with a chemical formula of CH2O. Carbon is in group for a oxygen is in group 6A and hydrogen is in group 1A on the periodic table. In 1 to 2 sentences describe the bonds in a molecule of formaldehyde in terms of valence electrons.

19.) Use the reaction to complete the activity.

KOH+HBr -> KBr+H2O

A student set up a titration apparatus with 25 mL Of potassium hydroxide in the flask. In 3 to 5 sentences explain how the student can use a 0.1 M Solution of hydrobromic acid to determine the concentration of potassium hydroxide in the reaction.

2 answers

  1. Here is a link that gives you the Lewis dot structure for CH2O. If that messes up then you can google it yourself. Here is the link.
    You have 4 electrons from C, 2 electrons from two H atoms and 6 electrons from O for a total of 12 electrons. You can write sentences that the 12 electrons are arranged so that C shares 8, O shares 8 and each H shares 2. From your post I'm not understanding what you aren't understanding. So if this doesn't get it please repost and explain more fully what you need help with.
    For #2. KOH + HBr -> KBr + H2O
    Frankly I don't understand why sentences are necessary when working a problem does a better job but here goes.
    In titrations in analytical chemistry (usually quantitative analysis) one wants to determine the concentration of one of the two regents. Usually that is in mole/L or molarity or M. In this case the problem states that we want to determine the M of KOH. When this happens we must have a KNOWN. In this case the known is HBr and we know the concentration is 0.1 M. So we know how much KOH we used (25 mL). We fill a buret with the 0.1 M HBr and add it to the KOH in the flask, with stirring, until we have EXACTLY neutralized the KOH. We can add an indicator that will tell us when the neutral point has been reached. Several indicators may be used but that's another story. When the neutral point has been reached we read the buret. Let's just say we used 20.00 mL of the HBr. The whole secret with quant is to make moles = mols and the indicator tells us when mols KOH = mols HBr. So how many mols HBr did we add Thats M x L = 20.00 x 0.1 M 2.000. That means we must have had 2.000 mols KOH in the flask initially. So if mols KOH = M x L and we know 2.000 moles KOH = M x 0.025 L then M KOH = 2.000/0.025 = 0.0800 M. Now you can summarize that in your own words. In simplest form, you are titration a measured amount of an unknown with a measured amount of a known in order to determine the strength of the unknown.

  2. DrBob, that was extremely helpful, and you covered what I didn’t understand, thank you so much! :)

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