acid base - What is the difference between these equations for the Contact process?

Some webpages describe the contact process as this (I left out $\ce{SO2}$, $\ce{SO3}$ production):

$\ce{SO3 + H2SO4 -> H2S2O7}$

After this, it is controllable to add water to the oleum therefore making liquid sulfuric acid. However, some other sites describe this:

$\ce{SO3 + H2O -> H2SO4}$

I am confused about what really happens in the absorption tower. I've read about the wet acid, but it states that adding water causes gaseous $\ce{H2SO4}$ to form, which is condensed.

EDIT: In the absorption tower, trioxide sulfur enters as gas, and sulfuric acid comes down as liquid. As a result the liquid comes out as oleum (first eq.). The other contact process states different: the liquid entering is 98% sulfuric acid, and 2% water, this water reacts with the sulfur trioxide (this little water avoids the heat problem), giving strengthened sulfuric acid (let's say 99.5% sulfuric acid, and 0.5% water). Still confused.

halacha - May one wear a tallit gadol with all four corners in front?

Qitzur Shulhhan Arukh - Yalqut Yosef (Orahh Hayim 8:26) states that one should wear one's tallit such that two corners are in front and two in back.

Nevertheless, I have encountered no small number of Teimani Jews (and Reform men/women or simply Jews of other persuasions) wearing a tallit with all four corners in front (e.g. Pic 1, Pic 2, Pic 3).

Does any Orthodox poseq consider this type of wearing (with all four corners/tzitziyot in front) eligible for fulfilling one's requirement (of placing tzitziyot on the four-cornered garment one wears)?

digital communications - How is symbol synchronization with OFDM done?

I'm trying to understand how symbol synchronization is done in OFDM using pilot tones, cyclic prefixes, or any other technique.

I've read the following answers which provide some explanation, but I still don't totally understand it.

How to demodulate an OFDM signal

How to estimate the number of taps needed for subsequent channel estimation algorithms?

Specific questions:

1) How is a pilot tone found? What makes it different than the regular data on a sub-carrier? How can it be used to determine symbol starts and ends?

2) If I understand the answers above correctly, a cyclic prefix can be used to find the symbol start/end because it will auto-correlate with some delay. However, the cyclic-prefix exists in order to "absorb" ISI. So if the prefix has been munged with ISI, then how can this auto-correlation be successful?

words - What color is Yarok?

Yarok ירוק in modern Hebrew means green. What color does it refer to in Mishnaic Hebrew? I have heard that it refers to yellow and not green because the Mishna in Sukkah (on 34b) discusses a citron which is "Yarok like a leek" implying that plain yarok is not like a leek, ie not green. ShmuelBrill in a comment says that Yarok sometimes means yellow and sometimes means green. It seems odd to me that one word would mean two colors. Can anyone explain?

halacha - Does Shmita apply in Jordan?

Since the East bank of the Jordan River was given to shevet Reuven as an inheritance, was Shmita observed there as well? If so, does shmita still apply there?

Answer

I'm just adding this answer due to the faulty translation in the accepted answer:

רמב"ם משנה תורה הלכות שמיטה ויובל

Rambam Mishneh Torah Laws of Shmita and Yovel

כג [כה] אין שביעית נוהגת אלא בארץ ישראל בלבד

Shmita applies only in the Land of Israel itself.

כה [כח] עבר הירדן, שביעית נוהגת בה מדבריהם.

On the eastern bank of the Jordan river, shmita is Rabbinic.

polarity - Why H2O2's molecular geometry is what it is...?

I came up with this question when I was trying to determine whether H2O2 is polar or not.

H2O2 looks like this when looked parallel to oxygen-oxygen bound:(source: Pubchem)

Each hydrogen is in one of the three positions available around each oxygen(the two nonbonding electron pairs occupy the other two); even the oxygen-oxygen bound can rotate to set hydrogens in any position relative to each other. Therefore I don't see the problem in following nonpolar structure(pretty neat!)

What concept am I missing while determining molecular geometry?

Is a H2O2 molecule sometimes polar and sometimes nonpolar?

Thanks for your help!

talmud gemara - What are some good modern Brisker/Chakira based analysis sefarim by masechta?

I'm a big fan of the Brisker/Chakira approach to gemara b'iyun and looking for a sefer that I can use to learn either alone or with a chavrusa.

I'm looking for something in Hebrew, and (a) by masechta, and (b) something more modern than the Briskers themselves.