Answer:
Four covalent bonds.
Explanation:
Hello,
In this case, given the attached picture in which you can find the Lewis dot structure for metanal (formaldehyde) we can see two C-H bonds and two C-O bonds via a double bond, thus, we can compute the type of each bond given the electronegativities of hydrogen, carbon and oxygen which are 2.1, 2.5 and 3.5 respectively:

Thus, since both electronegativity difference are less 1.7 we infer that all of them are covalent, therefore, it has four covalent bonds, two C-H bonds and a double C-O bond.
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Answer:
A positive ions is always smaller than the corresponding atom.
A negative ion is always larger than the corresponding atom.
Explanation:
The reason for this is that, when a positive ion is formed, a full shell is usually removed with its electrons thereby reducing the size of the electron cloud and decreasing the size of the electron cloud.
A negative ion is formed by addition of more electrons to the electron cloud hence it spreads out. Interelectronic repulsion accounts for the larger size of the negative ion.
We need to first come up with a balanced equation:
→ 
We know that the molar ratio of hydrogen to oxygen to water now is 4:1:2.
Converting the amount of grams given to moles is as follows:
Hydrogen: 
Oxygen: 
We know now that the limiting reactant is oxygen. We can then know that the number of moles of water are produced are double the number of moles of oxygen used due to the ratio that we established at the beginning - 4:1:2.
So we now can use 6.25 moles of water as the amount produced.
Then we convert moles of water to grams:

Now we know that there are 112.59g of water produced when we start with 50g of hydrogen and 50g of water.
What Can you be more clear --Molecules change it from being less dense